Add new test case to ensure git-merge reduces octopus parents when possible

The old shell version used show-branch --independent to filter for the
ones that cannot be reached from any other reference.

The new C version uses reduce_heads() from commit.c for this, so
add test to ensure it works as expected.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7603-merge-reduce-heads.sh

@@ -0,0 +1,63 @@
+#!/bin/sh
+
+test_description='git-merge
+
+Testing octopus merge when reducing parents to independent branches.'
+
+. ./test-lib.sh
+
+# 0 - 1
+#   \ 2
+#   \ 3
+#   \ 4 - 5
+#
+# So 1, 2, 3 and 5 should be kept, 4 should be avoided.
+
+test_expect_success 'setup' '
+	echo c0 > c0.c &&
+	git add c0.c &&
+	git commit -m c0 &&
+	git tag c0 &&
+	echo c1 > c1.c &&
+	git add c1.c &&
+	git commit -m c1 &&
+	git tag c1 &&
+	git reset --hard c0 &&
+	echo c2 > c2.c &&
+	git add c2.c &&
+	git commit -m c2 &&
+	git tag c2 &&
+	git reset --hard c0 &&
+	echo c3 > c3.c &&
+	git add c3.c &&
+	git commit -m c3 &&
+	git tag c3 &&
+	git reset --hard c0 &&
+	echo c4 > c4.c &&
+	git add c4.c &&
+	git commit -m c4 &&
+	git tag c4 &&
+	echo c5 > c5.c &&
+	git add c5.c &&
+	git commit -m c5 &&
+	git tag c5
+'
+
+test_expect_success 'merge c1 with c2, c3, c4, c5' '
+	git reset --hard c1 &&
+	git merge c2 c3 c4 c5 &&
+	test "$(git rev-parse c1)" != "$(git rev-parse HEAD)" &&
+	test "$(git rev-parse c1)" = "$(git rev-parse HEAD^1)" &&
+	test "$(git rev-parse c2)" = "$(git rev-parse HEAD^2)" &&
+	test "$(git rev-parse c3)" = "$(git rev-parse HEAD^3)" &&
+	test "$(git rev-parse c5)" = "$(git rev-parse HEAD^4)" &&
+	git diff --exit-code &&
+	test -f c0.c &&
+	test -f c1.c &&
+	test -f c2.c &&
+	test -f c3.c &&
+	test -f c4.c &&
+	test -f c5.c
+'
+
+test_done