Add abdi_repo to manage arcyd's repositories

Simplify abdcmd_addrepo by moving it's cloning logic out to a 'setup'
context manager that will automatically clean up the repository if any
of the setup steps fails.

Test Plan:
$ ./precommit.sh
WIP: Arcyd: add 'fsck' command for fixing up ...
fixup! WIP: Arcyd: add 'fsck' command for fixing up ...