Arcyd: archive abandoned branches after a warning

Help keep repositories tidy by automatically archiving review branches
associated with abandoned reviews.

Issue a warning on the abandoned review and wait until the review has
not recently been updated before automatic removal.

Give explicit commands for recovering the branch, using the real
branch name and SHA1.

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

change is_review_recently_updated() delay to 1 minute, land and
abandon reviews in
$ ./testbed/arcyd/test_shell.sh