Arcyd: archive landed branches to a special ref

Make it easier to deal with the sha1 discontinuity caused by squashing
by merging the 'originals' of landed branches into a special ref.

This ref is useful for:

- cleaning up branches contained by the landed branch
  (see 'git branch --merged')

- finding the pre-landed version of a commit
  (see 'git log --grep' - you can search for the landed sha1)

- keeping track of Arcyd's landing activity
  (see 'git log --first-parent')

Deprecate current implementation of 'barc gc'.

Update tests to exercise new cleanup using just Git commands.

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

Land and retire branches from
$ ./testbed/arcyd/test_shell.sh
Arcyd: update abandoned ones only when new commits

Early-out of updating reviews if they don't have new commits and they
are abandoned.

This will reduce the amount of time spent re-checking reviews if they
are in an abandoned state.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
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
Arcyd: test new abdt_conduit predicates

Exercise the new is_review_accepted() and is_review_abandoned() members
a little.

Test Plan:
$ ./precommit.sh
meta/autofix/fixlegal: only process existing files

Filter the list of files so we only process ones that exist on disk.

Test Plan:
$ ./precommit.sh
Arcyd: remove unused abdt_git.set_name_email

Remove an unused method and consequently an otherwise unused module
at the same time.

Test Plan:
$ ./precommit.sh

ensure is empty
$ git grep set_name_email
abdt_branch: use cheaper self._tracking_hash

Replace usage of a more expensive (and redundant) query with the
already stored result.

Test Plan:
$ ./precommit.sh