Arcyd: add-repo: fetch existing special refs

We want to continue the history of the 'landed' and 'abandoned' refs
if we're importing repositories into a new arcyd instance.

Check if the refs exist and if so then fetch them.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

land a branch and then create a new arcyd instance in
$ ./testbed/arcyd/test_shell.sh