mail.sh: further restrict new branch change summary
The recently added changes to improve the new branch change
summary when the new branch is a fork of a pre-existing revision
rely on the heuristic that the last shown revision by show_new_revisions
will be the single leaf in the (possibly partial) graph shown by
show_new_revisions.
This may not always be the case.
Therefore detect these other cases (where show_new_revisions showed a
graph fragment with more than one leaf) and suppress the summary of
changes for new branch notifications when this situation is detected
to avoid showing an incorrect and misleading summary of changes.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>