Handle forced updates, including rebases.  git-cherry is used to
determine if the commits pushed are present upstream (with a different
hash) or not.  Ones already present are excluded from the commit mail
since they haven't changed (i.e., they point to the same blobs the old
ones did).  Ones not present are either new commits or modified commits.
It's non-trivial to tell which is which, and we probably don't care
anyway.  Added text to 'forced-update' email messages to state that the
commits shown are only the new/modified ones and not all that were
pushed.