shortlog: prompt when reading from terminal by mistake

shortlog: prompt when reading from terminal by mistake

I was trying to see who have been active recently to find GSoC
mentor candidates by running:

	$ git shortlog -s -n --since=4.months | head -n 20

After waiting for about 20 seconds, I started getting worried,
thinking that the recent revision traversal updates might have
had an unintended side effect.

Not so.  "git shortlog" acts as a filter when no revs are given,
unlike "git log" which defaults to HEAD.  It was reading from
its standard input.

Signed-off-by: Junio C Hamano <junkio@cox.net>