Set `in_progress_merge_commit_sha` for fast-forward merges.

- Closes #900

1. This flag is used in conjunction with the "Developers can Merge"
   branch permission, to allow a user to merge MRs without push access
   to a branch.

2. Without this flag set, the user doesn't have access to the protected
   branch, and the merge fails.

3. This flag was previously set for the `Repository#merge` method, but
   not the `Repository#ff_merge` method.

See merge request !671