This replaces ad4617c95 with a different fix for the same issue:
instead of stripping the `.git` off separately, we can just correct
the regex, using `+?` to fix our stepping in a classic regex pitfall.
I've often done this by hand -- basically typed out the last line,
with the variables found from looking at the PR page in a browser.
Seems nicer for both us maintainers and the contributor, in particular
because the PR gets marked as merged, instead of closed. But it's a
bit of a pain, and I do it maybe half the time or less; plus it's kind
of a subtle GitHub feature, and as a result I think other maintainers
of Zulip repos do this approximately never.
I've always figured this couldn't be hard to automate; today I decided
to take the 45 minutes to look up how, write out the script, QA it,
write up a nice usage message and some comments, and commit it. :)
