Merge branch 'ps/connectivity-optim'

The revision traversal API has been optimized by taking advantage
of the commit-graph, when available, to determine if a commit is
reachable from any of the existing refs.

* ps/connectivity-optim:
  revision: avoid hitting packfiles when commits are in commit-graph
  commit-graph: split out function to search commit position
  revision: stop retrieving reference twice
  connected: do not sort input revisions
  revision: separate walk and unsorted flags