[Github] Add --diff_from_common_commit to code format repro command

This ensures that we are not including any branches on main that are not
in the current user's branch in the diff. We can add this to the command
now that --diff_from_common_commit (or at least the fixed version) has
landed in a release (21.1.1).

Author: boomanaiden154

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

@@ -205,9 +205,10 @@ def _construct_command(self, diff_expression: list[str] | None):
 
     @property
     def instructions(self) -> str:
-        # TODO(boomanaiden154): Add --diff_from_common_commit option when it has
-        # landed as in available in a released version.
-        return " ".join(self._construct_command(["origin/main", "HEAD"]))
+        return (
+            " ".join(self._construct_command(["origin/main", "HEAD"]))
+            + " --diff_from_common_commit"
+        )
 
     def should_include_extensionless_file(self, path: str) -> bool:
         return path.startswith("libcxx/include")