[CI][format] Explicitly pass extensions to git-clang-format (take 2)

This is a second attempt to land 7620fe0, which was reverted in 9572388
because it caused formatting not to be enforced for several patches.

llvm/utils/git/code-format-helper.py

@@ -216,6 +216,17 @@ def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str
             cf_cmd.append(args.start_rev)
             cf_cmd.append(args.end_rev)
 
+        # Gather the extension of all modified files and pass them explicitly to git-clang-format.
+        # This prevents git-clang-format from applying its own filtering rules on top of ours.
+        extensions = set()
+        for file in cpp_files:
+            _, ext = os.path.splitext(file)
+            extensions.add(
+                ext.strip(".")
+            )  # Exclude periods since git-clang-format takes extensions without them
+        cf_cmd.append("--extensions")
+        cf_cmd.append(",".join(extensions))
+
         cf_cmd.append("--")
         cf_cmd += cpp_files