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>

builtin/shortlog.c

@@ -11,7 +11,8 @@
 #include "parse-options.h"
 
 static char const * const shortlog_usage[] = {
-	N_("git shortlog [<options>] [<revision-range>] [[--] [<path>...]]"),
+	N_("git shortlog [<options>] [<revision-range>] [[--] <path>...]"),
+	N_("git log --pretty=short | git shortlog [<options>]"),
 	NULL
 };