Ensure the 'reserve' branch when adding repos

Make sure that repos added to arcyd have the 'dev/arcyd/reserve' branch
created. This wards off problems where a developer creates a 'dev'
branch and unwittingly blocks Arcyd from creating it's tracker branches
under 'dev/arcyd/'.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ git fetch
    $ git log origin/dev/arcyd/reserve