Fix abdi_processrepo: no branch name after landing

Branches won't have a review or tracking branch after landing, fix
abdi_processrepo to not ask for them after landing.

Add a test to abdi_processrepo__t for diffs that are too big.

Clear out the review and tracking branches in abdt_branch after landing,
update the branchmock to behave the same way.

Fix the abdi_processrepo__t merge conflict test to actually land the
review and test for null-ness afterwards.

Test Plan: $ ./precommit.sh