This commit migrates all webhooks to use check_send_stream_message
instead of check_send_message. The only two webhooks that still
use check_send_message are our yo and teamcity webhooks. They
both use check_send_message for private messages.
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.
For Git push messages, we now have a single space character between
the name of a commit's author and the number of commits by that
author, plus a period at the end.
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.
