rev-list: introduce --count option

Add a --count option that, instead of actually listing the commits,
merely counts them.

This is mostly geared towards script use, and to this end it acts
specially when used with --left-right: it outputs the left and right
counts separately.  Previously, scripts would have to run a shell loop
or small inline script over to achieve the same.  (Without
--left-right, a simple |wc -l does the job.)

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -57,6 +57,7 @@ struct rev_info {
 			limited:1,
 			unpacked:1,
 			boundary:2,
+			count:1,
 			left_right:1,
 			rewrite_parents:1,
 			print_parents:1,
@@ -131,6 +132,10 @@ struct rev_info {
 
 	/* notes-specific options: which refs to show */
 	struct display_notes_opt notes_opt;
+
+	/* commit counts */
+	int count_left;
+	int count_right;
 };
 
 #define REV_TREE_SAME		0