add commit count options to git-shortlog

This patch does 3 things:

1) Output the number of commits along with the name for each author
   (nice to know for long lists spending more than a screen worth of
   commit lines).

2) Provide a switch (-n) to sort authors according to their number of
   commits instead of author alphabetic order.

3) Provide a switch (-s) to supress commit lines and only keep a
   summary of authors and the number of commits for each of them.

And for good measure a short usage is displayed with -h.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>