mail.sh: suppress new branch diffstat for root+merge
Tweak the logic slightly so that if a new branch notification would
show the new branch all the way down to its root commit, that it does
not mistakenly show the diff stat when one of the commits on the way
to the root was a merge that merged in commits previously shown.
Basically this was just an off-by-1 error when showing a new branch
whose root commit had never been seen before.
Also fix the comments about what LAST_SHOWN_NBOUNDARY really is.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>