R1:65f13b156f5e

Improve "refengine" performance for testing large numbers of Mercurial branches

Summary:
See PHI158. In the RefEngine, we test if any old branch positions have been removed from the repository. This is uncommon (but not impossible) in Mercurial, and corresponds to users deleting branches in Git.

Currently, we end up running `hg log` for each position, in parallel. Because of Python's large startup overhead, this can be resource intensive for repositories with a large number of branches.

We have to do…