add GetsubmoduleDiff and GetSubmoduleStateFromIndex to CogWrapper

Without it, runnign gclient in Cog would fail with error
"AttributeError: 'CogWrapper' object has no attribute 'GetSubmoduleDiff'"

Change-Id: I6a3220f882eb38da9ebc59b6cac67743c41f4c7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5509813
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>

gclient_scm.py

@@ -2192,6 +2192,14 @@ class CogWrapper(SCMWrapper):
     def GetActualRemoteURL(self, options):
         return None
 
+    #override
+    def GetSubmoduleDiff(self):
+        return None
+
+    #override
+    def GetSubmoduleStateFromIndex(self):
+        return None
+
     #override
     def DoesRemoteURLMatch(self, options):
         del options