barc gc: check upstream landinglog before setup

Don't set up the local repo to fetch the upstream landinglog if it
doesn't exist yet.

There's no point in setting up the local repo to fetch the landinglog
ref if it doesn't exist, in fact this can present the user with some
very puzzling errors.

Test Plan:
poke, accept, gc in
$ ./testbed/arcyd/test_shell.sh
$ ./precommit.sh
Barc: exit with return code from subcommands

Test Plan:
exercise in $ ./testbed/arcyd/test_shell.sh
$ ./precommit.sh