Ref T1049. Ref T2015. This implements the "working copy" blueprint in Drydock and allows Harbormaster to lease from it. More importantly, this implements a cache for repositories, such that a cache of the remote repository is maintained per host resource, and subsequent working copy leases perform git fetch --all on the cache and clone locally.
Currently the working copy blueprint supports taking a lease based on buildablePHIDs, but could easily be extended to allow custom leases. It also resolves revision buildables to their diff / revision IDs, but does not yet support actually acquiring a lease based on a revision.