Clarify that gclient gitmodules' changes need to be committed.

Bug: 1483198
Change-Id: I1dfae06bfc273b01d168b507861984327ad0d629
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4892468
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
diff --git a/gclient.py b/gclient.py
index 388799f..ec633dc 100755
--- a/gclient.py
+++ b/gclient.py
@@ -2901,8 +2901,8 @@
     if cache_info:
         subprocess2.call(['git', 'update-index', '--add'] + cache_info)
     subprocess2.call(['git', 'add', '.gitmodules'])
-    print('.gitmodules and gitlinks updated. Please check git diff and '
-          'commit changes.')
+    print('.gitmodules and gitlinks updated. Please check `git diff --staged`'
+          'and commit those staged changes (`git commit` without -a)')
 
 
 @metrics.collector.collect_metrics('gclient flatten')