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.
Add detailed info (description, source and target branch, assignee) to message.
Change subject to 'repo_name / MR #id title'.
Modify some test fixtures for better coverage.
Fixes: #1883.