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>

Documentation/rev-list-options.txt

@@ -98,6 +98,15 @@ you would get an output like this:
 This implies the '--topo-order' option by default, but the
 '--date-order' option may also be specified.
 
+ifdef::git-rev-list[]
+--count::
+	Print a number stating how many commits would have been
+	listed, and suppress all other output.  When used together
+	with '--left-right', instead print the counts for left and
+	right commits, separated by a tab.
+endif::git-rev-list[]
+
+
 ifndef::git-rev-list[]
 Diff Formatting
 ~~~~~~~~~~~~~~~

builtin/rev-list.c

@@ -50,6 +50,15 @@ static void show_commit(struct commit *commit, void *data)
 
 	graph_show_commit(revs->graph);
 
+	if (revs->count) {
+		if (commit->object.flags & SYMMETRIC_LEFT)
+			revs->count_left++;
+		else
+			revs->count_right++;
+		finish_commit(commit, data);
+		return;
+	}
+
 	if (info->show_timestamp)
 		printf("%lu ", commit->date);
 	if (info->header_prefix)
@@ -400,5 +409,12 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 			     quiet ? finish_object : show_object,
 			     &info);
 
+	if (revs.count) {
+		if (revs.left_right)
+			printf("%d\t%d\n", revs.count_left, revs.count_right);
+		else
+			printf("%d\n", revs.count_left + revs.count_right);
+	}
+
 	return 0;
 }

revision.c

@@ -1146,6 +1146,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->boundary = 1;
 	} else if (!strcmp(arg, "--left-right")) {
 		revs->left_right = 1;
+	} else if (!strcmp(arg, "--count")) {
+		revs->count = 1;
 	} else if (!strcmp(arg, "--cherry-pick")) {
 		revs->cherry_pick = 1;
 		revs->limited = 1;

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

t/t6007-rev-list-cherry-pick-file.sh

@@ -32,6 +32,23 @@ test_expect_success setup '
 	git tag B
 '
 
+cat >expect <<EOF
+<tags/B
+>tags/C
+EOF
+
+test_expect_success '--left-right' '
+	git rev-list --left-right B...C > actual &&
+	git name-rev --stdin --name-only --refs="*tags/*" \
+		< actual > actual.named &&
+	test_cmp actual.named expect
+'
+
+test_expect_success '--count' '
+	git rev-list --count B...C > actual &&
+	test "$(cat actual)" = 2
+'
+
 test_expect_success '--cherry-pick foo comes up empty' '
 	test -z "$(git rev-list --left-right --cherry-pick B...C -- foo)"
 '
@@ -54,4 +71,16 @@ test_expect_success '--cherry-pick with independent, but identical branches' '
 		HEAD...master -- foo)"
 '
 
+cat >expect <<EOF
+1	2
+EOF
+
+# Insert an extra commit to break the symmetry
+test_expect_success '--count --left-right' '
+	git checkout branch &&
+	test_commit D &&
+	git rev-list --count --left-right B...D > actual &&
+	test_cmp expect actual
+'
+
 test_done