Adapt to upstream changes

- ChangeHooks is gone. Hooks are now invoked via the ref-update event.
- The ref-update firing mechanism now needs the user
- ProjectControl#canReadCommit now takes a Repository, not RevWalk
- Repository#getRef is deprecated in favor of #exactRef

The Buck build was already updated to build with 2.13-SNAPSHOT, but
the pom.xml was still using 2.12-SNAPSHOT, so update that too.

Change-Id: I3dd4b22c8ab3f8d12c0e787807e7fc1ca443c456