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MINOR: [Archery] Improve crossbow report-pr ergonomics (#13994)

* If a pull request with the given title is not found, also print the repo name.
* Allow shorthand "username/reponame" as remote, in addition to full Github URLs.

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>