We used to be very casual in terminology and used <branch>, <ref> and
<rev> more or less interchangeably with <commit>. Match the help text
given by "git merge -h" with that of the documentation.
Signed-off-by: Jared Hance <jaredhance@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>