Add tests for shallow
Determine new shallow/unshallow
Add shallow support to ProtocolGraphWalker
Add shallow grafts to repo before fetching objects
Specify custom get_parents
Look for shallow graft file in new Repos
Cleanup temporary repos in compat test
Simplify use of getattr.
Pass in commit during shallow clone, to avoid extra lookup.
Simplify shallows handling.
Simplify handling of shallows, without involving temporary grafts.
Merge milki's shallow branch support.