Arcyd: test no commits on branch case

Exercise path where there are no commits on the review branch.

Test Plan: $ ./precommit.sh
Arcyd: make branch and mock more alike

Make the abdt_branch and abdt_branchmock work more similarly when
abandoning a review.

Test the processrepo in the case where a branch has been deleted and
Arcyd needs to forget it.

Test Plan: $ ./precommit.sh