Never replicate automerge-cache commits

Whenever a change is diffed in the UI, the merge result is stored
as a commit under the refs/automerge-cache namespace in the related
git.  These commits are then replicated, which is not necessary.

Add checks to prevent these commits from being replicated.

Change-Id: Ia1896a822a9901918d318d7641e2e8b63a0cf355