Arcyd: comment on reviews using full commit hashes

Include full commit hash of review branch when creating,
updating or landing reviews.

This makes it much easier to reconstruct history, without having to
resort to any separate audit logs.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

Create, update and land review, verifying that hashes are as expected
$ ./testbed/arcyd/test_shell.sh