Update automatic merging documentation

We're moving from merging + reverting to only merging with the `ours`
strategy, removing the need for merge requests or reverting conflicts.
For more details, refer to Merge Train merge request
https://gitlab.com/gitlab-org/merge-train/merge_requests/6.

doc/development/automatic_ce_ee_merge.md

 # Automatic CE->EE merge
 
-Whenever a commit is pushed to the CE `master` branch, it is automatically
-merged into the EE `master` branch. If the commit produces any conflicts, it is
-instead reverted from CE `master`. When this happens, a merge request will be
-set up automatically that can be used to reinstate the changes. This merge
-request will be assigned to the author of the conflicting commit, or the merge
-request author if the commit author could not be associated with a GitLab user.
-If no author could be found, the merge request is assigned to a random member of
-the Delivery team. It is then up to this team member to figure out who to assign
-the merge request to.
-
-Because some commits can not be reverted if new commits depend on them, we also
-run a job periodically that processes a range of commits and tries to merge or
-revert them. This should ensure that all commits are either merged into EE
-`master`, or reverted, instead of just being left behind in CE.
+Commits pushed to CE `master` are automatically merged into EE `master` roughly
+every 5 minutes. Changes are merged using the `ours` merge strategy in the
+context of EE. This means that any merge conflicts are resolved by taking the EE
+changes and discarding the CE changes. This removes the need for resolving
+conflicts or reverting changes, at the cost of **absolutely requiring** EE merge
+requests to be created whenever a CE merge request causes merge conflicts.
+Failing to do so can result in changes not making their way into EE.
+
+## Always create an EE merge request if there are conflicts
+
+In CI there is a job called `ee_compat_check`, which checks if a CE MR causes
+merge conflicts with EE. If this job reports conflicts, you **must** create an
+EE merge request. If you are an external contributor you can ask the reviewer to
+do this for you.
 
 ## Always merge EE merge requests before their CE counterparts
 
 **In order to avoid conflicts in the CE->EE merge, you should always merge the
 EE version of your CE merge request first, if present.**
 
-The rationale for this is that as CE->EE merges are done automatically, it can
-happen that:
-
-1. A CE merge request that needs EE-specific changes is merged.
-1. The automatic CE->EE merge happens.
-1. Conflicts due to the CE merge request occur since its EE merge request isn't
-   merged yet.
-1. The CE changes are reverted.
+Failing to do so will lead to CE changes being discarded when merging into EE,
+if they cause merge conflicts.
 
 ## Avoiding CE->EE merge conflicts beforehand
 
@@ -45,76 +39,184 @@ detect if the current branch's changes will conflict during the CE->EE merge.
 The job reports what files are conflicting and how to set up a merge request
 against EE.
 
-## How to reinstate changes
-
-When a commit is reverted, the corresponding merge request to reinstate the
-changes will include all the details necessary to ensure the changes make it
-back into CE and EE. However, you still need to manually set up an EE merge
-request that resolves the conflicts.
+#### How the job works
+
+1. Generates the diff between your branch and current CE `master`
+1. Tries to apply it to current EE `master`
+1. If it applies cleanly, the job succeeds, otherwise...
+1. Detects a branch with the `ee-` prefix or `-ee` suffix in EE
+1. If it exists, generate the diff between this branch and current EE `master`
+1. Tries to apply it to current EE `master`
+1. If it applies cleanly, the job succeeds
+
+In the case where the job fails, it means you should create an `ee-<ce_branch>`
+or `<ce_branch>-ee` branch, push it to EE and open a merge request against EE
+`master`.
+At this point if you retry the failing job in your CE merge request, it should
+now pass.
+
+Notes:
+
+- This task is not a silver-bullet, its current goal is to bring awareness to
+  developers that their work needs to be ported to EE.
+- Community contributors shouldn't be required to submit merge requests against
+  EE, but reviewers should take actions by either creating such EE merge request
+  or asking a GitLab developer to do it **before the merge request is merged**.
+- If you branch is too far behind `master`, the job will fail. In that case you
+  should rebase your branch upon latest `master`.
+- Code reviews for merge requests often consist of multiple iterations of
+  feedback and fixes. There is no need to update your EE MR after each
+  iteration. Instead, create an EE MR as soon as you see the
+  `ee_compat_check` job failing. After you receive the final approval
+  from a Maintainer (but **before the CE MR is merged**) update the EE MR.
+  This helps to identify significant conflicts sooner, but also reduces the
+  number of times you have to resolve conflicts.
+- Please remember to
+  [always have your EE merge request merged before the CE version](#always-merge-ee-merge-requests-before-their-ce-counterparts).
+- You can use [`git rerere`](https://git-scm.com/docs/git-rerere)
+  to avoid resolving the same conflicts multiple times.
+
+### Cherry-picking from CE to EE
+
+For avoiding merge conflicts, we use a method of creating equivalent branches
+for CE and EE. If the `ee-compat-check` job fails, this process is required.
+
+This method only requires that you have cloned both CE and EE into your computer.
+If you don't have them yet, please go ahead and clone them:
+
+- Clone CE repo: `git clone git@gitlab.com:gitlab-org/gitlab-ce.git`
+- Clone EE repo: `git clone git@gitlab.com:gitlab-org/gitlab-ee.git`
+
+And the only additional setup we need is to add CE as remote of EE and vice-versa:
+
+- Open two terminal windows, one in CE, and another one in EE:
+  - In EE: `git remote add ce git@gitlab.com:gitlab-org/gitlab-ce.git`
+  - In CE: `git remote add ee git@gitlab.com:gitlab-org/gitlab-ee.git`
+
+That's all setup we need, so that we can cherry-pick a commit from CE to EE, and
+from EE to CE.
+
+Now, every time you create an MR for CE and EE:
+
+1. Open two terminal windows, one in CE, and another one in EE
+1. In the CE terminal:
+   1. Create the CE branch, e.g., `branch-example`
+   1. Make your changes and push a commit (commit A)
+   1. Create the CE merge request in GitLab
+1. In the EE terminal:
+   1. Create the EE-equivalent branch ending with `-ee`, e.g.,
+      `git checkout -b branch-example-ee`
+   1. Fetch the CE branch: `git fetch ce branch-example`
+   1. Cherry-pick the commit A: `git cherry-pick commit-A-SHA`
+   1. If Git prompts you to fix the conflicts, do a `git status`
+      to check which files contain conflicts, fix them, save the files
+   1. Add the changes with `git add .` but **DO NOT commit** them
+   1. Continue cherry-picking: `git cherry-pick --continue`
+   1. Push to EE: `git push origin branch-example-ee`
+1. Create the EE-equivalent MR and link to the CE MR from the
+description "Ports [CE-MR-LINK] to EE"
+1. Once all the jobs are passing in both CE and EE, you've addressed the
+feedback from your own team, and got them approved, the merge requests can be merged.
+1. When both MRs are ready, the EE merge request will be merged first, and the
+CE-equivalent will be merged next.
+
+**Important notes:**
+
+- The commit SHA can be easily found from the GitLab UI. From a merge request,
+open the tab **Commits** and click the copy icon to copy the commit SHA.
+- To cherry-pick a **commit range**, such as [A > B > C > D] use:
+
+    ```shell
+    git cherry-pick "oldest-commit-SHA^..newest-commit-SHA"
+    ```
+
+    For example, suppose the commit A is the oldest, and its SHA is `4f5e4018c09ed797fdf446b3752f82e46f5af502`,
+    and the commit D is the newest, and its SHA is `80e1c9e56783bd57bd7129828ec20b252ebc0538`.
+    The cherry-pick command will be:
+
+    ```shell
+    git cherry-pick "4f5e4018c09ed797fdf446b3752f82e46f5af502^..80e1c9e56783bd57bd7129828ec20b252ebc0538"
+    ```
+
+- To cherry-pick a **merge commit**, use the flag `-m 1`. For example, suppose that the
+merge commit SHA is `138f5e2f20289bb376caffa0303adb0cac859ce1`:
+
+    ```shell
+    git cherry-pick -m 1 138f5e2f20289bb376caffa0303adb0cac859ce1
+    ```
+- To cherry-pick multiple commits, such as B and D in a range [A > B > C > D], use:
+
+    ```shell
+    git cherry-pick commmit-B-SHA commit-D-SHA
+    ```
+
+    For example, suppose commit B SHA = `4f5e4018c09ed797fdf446b3752f82e46f5af502`,
+    and the commit D SHA = `80e1c9e56783bd57bd7129828ec20b252ebc0538`.
+    The cherry-pick command will be:
+
+    ```shell
+    git cherry-pick 4f5e4018c09ed797fdf446b3752f82e46f5af502 80e1c9e56783bd57bd7129828ec20b252ebc0538
+    ```
+
+    This case is particularly useful when you have a merge commit in a sequence of
+    commits and you want to cherry-pick all but the merge commit.
+
+- If you push more commits to the CE branch, you can safely repeat the procedure
+to cherry-pick them to the EE-equivalent branch. You can do that as many times as
+necessary, using the same CE and EE branches.
+- If you submitted the merge request to the CE repo and the `ee-compat-check` job passed,
+you are not required to submit the EE-equivalent MR, but it's still recommended. If the
+job failed, you are required to submit the EE MR so that you can fix the conflicts in EE
+before merging your changes into CE.
+
+---
+
+[Return to Development documentation](README.md)
 
-Each merge request used to reinstate changes will have the "reverted" label
-applied. Please do not remove this label, as it will be used to determine how
-many times commits are reverted and how long it takes to reinstate the changes.
-
-An example merge request can be found in [CE merge request
-23280](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23280).
+## FAQ
 
-## How it works
+### How does automatic merging work?
 
 The automatic merging is performed using a project called [Merge
-Train](https://gitlab.com/gitlab-org/merge-train/). For every commit to merge or
-revert, we generate patches using `git format-patch` which we then try to apply
-using `git am --3way`. If this succeeds we push the changes to EE, if this fails
-we decide what to do based on the failure reason:
-
-1. If the patch could not be applied because it was already applied, we just
-   skip it.
-1. If the patch caused conflicts, we revert the source commits.
-
-Commits are reverted in reverse order, ensuring that if commit B depends on A,
-and both conflict, we first revert B followed by reverting A.
+Train](https://gitlab.com/gitlab-org/merge-train/). This project will clone CE
+and EE master, and merge CE master into EE master using `git merge
+--strategy=ours`. This process runs roughly every 5 minutes.
 
-## FAQ
-
-### Why?
+For more information on the exact implementation you can refer to the source
+code.
 
-We want to work towards being able to deploy continuously, but this requires
-that `master` is always stable and has all the changes we need. If CE `master`
-can not be merged into EE `master` due to merge conflicts, this prevents _any_
-change from CE making its way into EE. Since GitLab.com runs on EE, this
-effectively prevents us from deploying changes.
+### Why merge automatically?
 
-Past experiences and data have shown that periodic CE to EE merge requests do
-not scale, and often take a very long time to complete. For example, [in this
+As we work towards continuous deployments and a single repository for both CE
+and EE, we need to first make sure that all CE changes make their way into CE as
+fast as possible. Past experiences and data have shown that periodic CE to EE
+merge requests do not scale, and often take a very long time to complete. For
+example, [in this
 comment](https://gitlab.com/gitlab-org/release/framework/issues/49#note_114614619)
 we determined that the average time to close an upstream merge request is around
 5 hours, with peaks up to several days. Periodic merge requests are also
 frustrating to work with, because they often include many changes unrelated to
 your own changes.
 
-Automatically merging or reverting commits allows us to keep merging changes
-from CE into EE, as we never have to wait hours for somebody to resolve a set of
-merge conflicts.
-
-### Does the CE to EE merge take into account merge commits?
-
-No. When merging CE changes into EE, merge commits are ignored.
+To resolve these problems, we now merge changes using the `ours` strategy to
+automatically resolve merge conflicts. This removes the need for resolving
+conflicts in a periodic merge request, and allows us to merge changes from CE
+into EE much faster.
 
-### My changes are reverted, but I set up an EE MR to resolve conflicts
+### My CE merge request caused conflicts after it was merged. What do I do?
 
-Most likely the automatic merge job ran before the EE merge request was merged.
-If this keeps happening, consider reporting a bug in the [Merge Train issue
-tracker](https://gitlab.com/gitlab-org/merge-train/issues).
+If you notice this, you should set up an EE merge request that resolves these
+conflicts as **soon as possible**. Failing to do so can lead to your changes not
+being available in EE, which may break tests. This in turn would prevent us from
+being able to deploy.
 
-### My changes keep getting reverted, and this is really annoying!
+### Won't this setup be risky?
 
-This is understandable, but the solution to this is fairly straightforward:
-simply set up an EE merge request for every CE merge request, and resolve your
-conflicts before the changes are reverted.
-
-### Will we allow certain people to still merge changes, even if they conflict?
-
-No.
+No, not if there is an EE merge request for every CE merge request that causes
+conflicts _and_ that EE merge request is merged first. In the past we may have
+been a bit more relaxed when it comes to enforcing EE merge requests, but to
+enable automatic merging have to start requiring such merge requests even for
+the smallest conflicts.
 
 ### Some files I work with often conflict, how can I best deal with this?
 
@@ -123,11 +225,3 @@ so that the EE specific changes are not intertwined with CE code. For Ruby code
 you can do this by moving the EE code to a separate module, which can then be
 injected into the appropriate classes or modules. See [Guidelines for
 implementing Enterprise Edition features](ee_features.md) for more information.
-
-### Will changelog entries be reverted automatically?
-
-Only if the changelog was added in the commit that was reverted. If a changelog
-entry was added in a separate commit, it is possible for it to be left behind.
-Since changelog entries are related to the changes in question, there is no real
-reason to commit the changelog separately, and as such this should not be a big
-problem.