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 1aa3530314)

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

Tools/scripts/patchcheck.py

@@ -78,11 +78,14 @@ def get_git_remote_default_branch(remote_name):
     It is typically called 'main', but may differ
     """
     cmd = "git remote show {}".format(remote_name).split()
+    env = os.environ.copy()
+    env['LANG'] = 'C'
     try:
         remote_info = subprocess.check_output(cmd,
                                               stderr=subprocess.DEVNULL,
                                               cwd=SRCDIR,
-                                              encoding='UTF-8')
+                                              encoding='UTF-8',
+                                              env=env)
     except subprocess.CalledProcessError:
         return None
     for line in remote_info.splitlines():