Add alternative merge type and merge notes to merge requests

Created by: calmh

This adds two minor, optional features to merge requests that were necessary for our workflow and might be useful for others.

"merge_ff_only": Instead of the default "pull --no-ff" that creates a merge commit, do "merge --ff-only" on merge requests. This means the resulting history on the destination branch will be linear, and also that non-ff merge requests will be marked as unmergeable until rebased.

"add_merge_notes": With the above strategy we won't get the merge commit indicating who did the merge. Instead, this adds notes to refs/notes/merge to each merged commit with the text "Merge-By: Full Name email@example.com" which can be pulled and examined after the fact.

I'll be keeping this as a local patch either way, so if there's no interest from the community for these features, feel free to close the pull request.