git_drover: Replace with instructions on how to cherry-pick.

Change-Id: I4cf7591df2b89bceb5618abb8fad92fd29e066bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2653952
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Andy Perelson <ajp@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>