phlgitu_ref: add is_fq_local_branch()

Add a new Git ref predicate for determining if a given ref is a fully
qualified local branch name.

Test Plan:
$ ./precommit.sh
WIP: Add new command: 'barc list'

Introduce and new command to the Arcyd command-line helper tool for
listing out all the review branches being managed by Arcyd and their
status.

Test Plan:
$ ./precommit.sh
Run 'barc list' in a repo with many active reviews and validate output.