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>

builtin-shortlog.c

@@ -304,8 +304,11 @@ int cmd_shortlog(int argc, const char **argv, const char *prefix)
 	if (!access(".mailmap", R_OK))
 		read_mailmap(".mailmap");
 
-	if (rev.pending.nr == 0)
+	if (rev.pending.nr == 0) {
+		if (isatty(0))
+			fprintf(stderr, "(reading log to summarize from standard input)\n");
 		read_from_stdin(&list);
+	}
 	else
 		get_from_rev(&rev, &list);