Implement UpstreamBranch in Change class

This method was moved to GitChange in https://crrev.com/c/5286636 but
should be implemented for all Change classes for CheckPatchFormatted.

Bug: 323243527
Change-Id: Iafdf8be2f402a93cfe7285cb5e7d0d1e01cca049
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5332873
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

presubmit_support.py

@@ -1332,6 +1332,13 @@ class Change(object):
     def TBR(self):
         return ','.join(self.TBRsFromDescription())
 
+    def UpstreamBranch(self):
+        """Returns the upstream branch for the change.
+
+        This is only applicable to Git changes.
+        """
+        return None
+
     def AllFiles(self, root=None):
         """List all files under source control in the repo."""
         raise NotImplementedError()