Add missing --no-ext-diff when getting external changes.

Bug: 1441663
Change-Id: I66fc01e486ddddb6b8c296d1ed4faf22141b845e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4859062
Auto-Submit: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git_cl.py

@@ -3384,8 +3384,10 @@ class Changelist(object):
                 'Continue with upload and override the latest changes?')
             return
 
-        diff = RunGitSilent(
-            ['diff', '%s..%s' % (last_uploaded, latest_external)])
+        diff = RunGitSilent([
+            'diff', '--no-ext-diff',
+            '%s..%s' % (last_uploaded, latest_external)
+        ])
 
         # Diff can be empty in the case of trivial rebases.
         if not diff: