shortlog: add usage-string for stdin-reading

shortlog: add usage-string for stdin-reading

This has been missing since we learned to print usage, way back in
4e27fb06f (add commit count options to git-shortlog, 2006-10-06).

While at it, drop the [] around "<path>...". This matches `git log -h`
and Documentation/git-{short}log.txt. It formally makes it look like we
do not allow `git shortlog --`, but we gain readability and consistency.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>