R1:7b50b2fbdc73

Use subprocess output streaming to improve performance of Git commit discovery

Summary:
Improve performance of large discovery tasks in Git by using subprocess streaming, like we do for Mercurial.

Basically, we save the cost of running many `git log` commands by running one big `git log` command but only parsing as much of it as we need to. This is pretty complicated, but we more or less need it for mercurial (which has ~100ms of 'hg' overhead instead of ~5ms of 'git' overhead) so we're already committed…