Arcyd: maintain log of landed branches for gc'ing

Make it possible to for users to safely clean up landed branches locally
by maintaining a log of all the branches which have been landed and
their sha1's before being squashed on to the base branch.

In later work we can write a simple client app to do the actual clean
up.

Test Plan:
added breathing test
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh