Git operations cleanup

See merge request !1151

* ce/fix-git-hooks-when-creating-file: (78 commits)
  Make sure different project gets a merge request
  Try to check if branch diverged explicitly
  We don't care about the return value now
  Make GitHooksService#execute return block value
  Use commit rather than branch, and rename to avoid confusion
  @tree_edit_project is no longer used
  Fix renaming
  Rename from base to start because base could mean merge base
  Detect if we really want a new merge request properly
  Properly fix the edge case!
  Rename source to base to avoid confusion from MR
  Don't set invalid @mr_source_branch when create_merge_request?
  Prefer leading dots over trailing dots
  Fix for initial commit and remove unneeded args
  Just trust set_commit_variables to set everything!
  Prefer leading dots over trailing dots
  Merge request terms are reversed for GitOperationService
  I think I am really confused, should be @tree_edit_project
  Remove tag with git hooks
  Introduce Repository#with_repo_branch_commit
  ...

Add username and password to Jenkins hook url

Closes #804

See merge request !1035

Allow squashing MRs

Closes #150

See merge request !1024

Update guidance on handling shared secrets in HA setup

See merge request !1152

Snippet spam EE

See merge request !1148

Apply the same spam checks to public snippets (either personal snippets
that are public, or public snippets on public projects) as to issues on
public projects.

When a snippet is submitted, but there's an error, we didn't keep the
visibility level. As the default is private, this means that submitting
a public snippet that failed would then fall back to being a private
snippet.

Resolve ""Merge when pipeline succeeds" visible if approvals are pending"

Closes #1595

See merge request !1101

Add info on using both push/pull repository mirroring

See merge request !1146

[ci skip]

This avoids the need to:

1. Perform a full clone.
2. Deal with protected branches (because there is no push).
3. Deal with repository push events (because there is no push).

1. When a branch is protected by a wildcard, it can't be removed by
   anyone. Skip hooks on branch removal after squashing so that the
   branch is always removed.
2. Pushing a branch creates an event, cluttering the activity
   stream. Manually remove this, too.

If the temporary branch we want to push to is protected by a wildcard,
we need to add a temporary protected branch entry to allow the current
user to push to it, even if they can't administer the project.

In this case, we can save a lot of time and gymnastics by just
immediately returning the commit SHA. If it can't be merged, the
squashed commit couldn't be merged either.

* 'Learn more' should be 'about this feature'
* The feature is to squash and merge at once, not squash before merge