Merge branch 'docs-add-debug-git-tricks' into 'master'

Migrate Git tricks doc from /debug to docs and make some edits

See merge request gitlab-org/gitlab-ce!27922

doc/topics/git/index.md

@@ -48,6 +48,7 @@ The following are resources about version control concepts:
 
 The following resources may help you become more efficient at using Git:
 
+- [Useful Git commands](useful_git_commands.md) collected by the GitLab support team.
 - [Git Tips & Tricks](https://about.gitlab.com/2016/12/08/git-tips-and-tricks/)
 - [Eight Tips to help you work better with Git](https://about.gitlab.com/2015/02/19/8-tips-to-help-you-work-better-with-git/)
 

doc/topics/git/useful_git_commands.md

+---
+type: reference
+---
+
+# Useful Git commands
+
+Here are some useful Git commands collected by the GitLab support team. You may not
+need to use often, but they can can come in handy when needed.
+
+## Remotes
+
+### Add another URL to a remote, so both remotes get updated on each push
+
+```sh
+git remote set-url --add <remote_name> <remote_url>
+```
+
+## Staging and reverting changes
+
+### Remove last commit and leave the changes in unstaged
+
+```sh
+git reset --soft HEAD^
+```
+
+### Unstage a certain number of commits from HEAD
+
+To unstage 3 commits, for example, run:
+
+```sh
+git reset HEAD^3
+```
+
+### Unstage changes to a certain file from HEAD
+
+```sh
+git reset <filename>
+```
+
+### Revert a file to HEAD state and remove changes
+
+There are two options to revert changes to a file:
+
+- `git checkout <filename>`
+- `git reset --hard <filename>`
+
+### Undo a previous commit by creating a new replacement commit
+
+```sh
+git revert <commit-sha>
+```
+
+### Create a new message for last commit
+
+```sh
+git commit --amend
+```
+
+### Add a file to the last commit
+
+```sh
+git add <filename>
+git commit --amend
+```
+
+Append `--no-edit` to the `commit` command if you do not want to edit the commit
+message.
+
+## Stashing
+
+### Stash changes
+
+```sh
+git stash save
+```
+
+The default behavor of `stash` is to save, so you can also use just:
+
+```sh
+git stash
+```
+
+### Unstash your changes
+
+```sh
+git stash apply
+```
+
+### Discard your stashed changes
+
+```sh
+git stash drop
+```
+
+### Apply and drop your stashed changes
+
+```sh
+git stash pop
+```
+
+## Refs and Log
+
+### Use reflog to show the log of reference changes to HEAD
+
+```sh
+git reflog
+```
+
+### Check the Git history of a file
+
+The basic command to check the git history of a file:
+
+```sh
+git log <file>
+```
+
+If you get this error message:
+
+```text
+fatal: ambiguous argument <file_name>: unknown revision or path not in the working tree.
+Use '--' to separate paths from revisions, like this:
+```
+
+Use this to check the Git history of the file:
+
+```sh
+git log -- <file>
+```
+
+### Find the tags that contain a particular SHA
+
+```sh
+git tag --contains <sha>
+```
+
+### Check the content of each change to a file
+
+```sh
+gitk <file>
+```
+
+### Check the content of each change to a file, follows it past file renames
+
+```sh
+gitk --follow <file>
+```
+
+## Debugging
+
+### Use a custom SSH key for a git command
+
+```text
+GIT_SSH_COMMAND="ssh -i ~/.ssh/gitlabadmin" git <command>
+```
+
+### Debug cloning
+
+With SSH:
+
+```text
+GIT_SSH_COMMAND="ssh -vvv" git clone <git@url>
+```
+
+With HTTPS:
+
+```text
+GIT_TRACE_PACKET=1 GIT_TRACE=2 GIT_CURL_VERBOSE=1 git clone <url>
+```
+
+## Rebasing
+
+### Rebase your branch onto master
+
+The -i flag stands for 'interactive':
+
+```sh
+git rebase -i master
+```
+
+### Continue the rebase if paused
+
+```sh
+git rebase --continue
+```
+
+### Use git rerere
+
+To _reuse_ recorded solutions to the same problems when repeated:
+
+```sh
+git rerere
+```
+
+To enable `rerere` functionality:
+
+```sh
+git config --global rerere.enabled true
+```
+
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->