Add exact git update-index command in presubmit mismatch message.

Bug:1417051
Change-Id: Iaadced083d49de2e76c04649ab71a4b0534c0706
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4574568
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>

presubmit_canned_checks.py

@@ -1774,8 +1774,10 @@ def CheckForCommitObjects(input_api, output_api):
           output_api.PresubmitError(
               'DEPS file indicates git submodule migration is in progress,\n'
               'but the commit objects do not match DEPS entries.\n'
-              'Update the following commit objects or DEPS entries:\n',
-              mismatch_entries)
+              'Update the following commit objects with:\n'
+              '`git update-index --add --cacheinfo 160000,<commit_rev>,<path>`'
+              '\n'
+              'or DEPS entries:\n', mismatch_entries)
       ]
 
   return []