rev-list: allow -n<n> as shorthand for --max-count=<n>

Both -n<n> and -n <n> are supported.  POSIX versions of head(1) and
tail(1) allow their line limits to be parsed this way.  I find
--max-count to be a commonly used option, and also similar in spirit to
head/tail, so I decided to make life easier on my worn out (and lazy :)
fingers with this patch.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>