bpo-44125: Fix "make patchcheck" on non-English locale (GH-26102)

The patch from [bpo-44074]() does not account for a possibly non-English locale and blindly greps for "HEAD branch" in a possibly localized text.

Automerge-Triggered-By: GH:pitrou
(cherry picked from commit 1aa3530314d339725519f4ad95b7dea4b00b657e)

Co-authored-by: Antoine Pitrou <antoine@python.org>