DEV: patchbot: update: report git commit failures in the save response

DEV: patchbot: update: report git commit failures in the save response

When the git commit fails after a save (typically a missing committer
identity in the storage repository, or an ownership/permission issue),
the failure was only logged to stderr, which lands in the web server's
error log at best: the file kept being updated but the history silently
stopped being recorded. Report it as a "warning: git commit failed ..."
line appended to the response, where the page can show it to the user,
on top of the stderr log. Also stop treating a no-op as a failure:
re-pushing identical content stages nothing, so the commit is now
simply skipped when "git diff --cached --quiet" reports no staged
change, instead of letting "git commit" fail on an empty commit.