Make PullLocal smart about which repositories it should pull

Summary:
Ref T10756. When repositories are properly configured for the cluster (which is hard to set up today), be smart about which repositories are expected to exist on the current host, and only pull them.

This generally allows daemons to pretty much do the right thing no matter how many copies are running, although there may still be some lock contention issues that need to be sorted out.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10756

Differential Revision: https://secure.phabricator.com/D15682