1. When a merge request is being merged, save the merge commit SHA in
the `in_progress_merge_commit_sha` database column.
2. The `pre-receive` hook looks for any locked (in progress) merge
request with `in_progress_merge_commit_sha` matching the `newrev` it
is passed.
3. If it finds a matching MR, the merge is legitimate.
4. Update `git_access_spec` to test the behaviour we added here. Also
refactored this spec a bit to make it easier to add more contexts / conditions.
