resubmit creates a new Change-Id for the current change and thus allow to submit
changes for review on release branches that were already reviewed on master.

Also hint advanced users to 'git review' which requires some more setup, but
should make things easier for regular users.

Getting this functionality into 'git review' would be likely help adoption too
-- and in python there is some sane errorchecking possible.

Change-Id: Ibea6bbfe747af160728b838c6ee236fd8f89671d