testbed/arcyd: no need to fetch landinglog

Don't explicitly fetch the landinglog as 'barc' will do this for us as
necessary.

Do checkout onto master before running barc, it cannot remove the
current branch and we may be on a moribund branch.

Test Plan:
$ ./smoke_tests.sh
$ ./testbed/arcyd/acceptance_tests.sh
arcyd: don't push landinglog

Disable the pushing of the landinglog for now - some repos don't allow
us to write to the ref.

Perhaps we'll add a config option that will allow this later.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
poke, accept, fail to barc in
$ ./testbed/arcyd/test_shell.sh