Suggest --no-fetch flag after git rebase-update conflict

This spares an unnecessary git fetch.

Change-Id: I552148bdfadc3482bda0d71d75aaf9d9ed61d75d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4062088
Auto-Submit: Victor Vianna <victorvianna@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

git_rebase_update.py

@@ -208,7 +208,7 @@ def rebase_branch(branch, parent, start_hash):
                  git rebase --abort && \\
                  git config branch.%s.dormant true
 
-          And then run `git rebase-update` again to resume.
+          And then run `git rebase-update -n` to resume.
           """ % branch))
           return False
   else: