For our Git integrations, we now only display the number of commits
pushed when the pusher also happens to be the only author of the
commits being pushed.
Part of #3968.
Follow-up to #4006.
We now show a few new things:
(1) The number of commits pushed.
(2) Who authored the commits (just counts, not which specific ones, for brevity).
Add tests for case of multiple committers.
Part of #3968.