Implement it, like it is described in git-shortlog.

Signed-off-by: Florian Mickler <florian@mickler.org>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>