[wpt] Discard stderr when calling git

git occasionally prints warning messages to stderr (e.g. when too many
files are modified and rename detection is disabled), which would mess
with the output parsing if they are redirected to stdout.

Fixes #18608.

tools/wpt/testfiles.py

@@ -51,7 +51,7 @@ def git(cmd, *args):
         full_cmd = [u"git", cmd] + list(item.decode("utf8") if isinstance(item, bytes) else item for item in args)  # type: List[Text]
         try:
             logger.debug(" ".join(full_cmd))
-            return subprocess.check_output(full_cmd, cwd=repo_path, stderr=subprocess.STDOUT).decode("utf8").strip()
+            return subprocess.check_output(full_cmd, cwd=repo_path, stderr=None).decode("utf8").strip()
         except subprocess.CalledProcessError as e:
             logger.error("Git command exited with status %i" % e.returncode)
             logger.error(e.output)