Add testcase to ensure merging an early part of a branch is done properly

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

t/t7600-merge.sh

@@ -507,4 +507,30 @@ test_expect_success 'in-index merge' '
 
 test_debug 'gitk --all'
 
+cat >expected <<EOF
+Merge branch 'c5' (early part)
+EOF
+
+test_expect_success 'merge early part of c2' '
+	git reset --hard c3 &&
+	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 &&
+	git reset --hard c3 &&
+	echo c6 > c6.c &&
+	git add c6.c &&
+	git commit -m c6 &&
+	git tag c6 &&
+	git merge c5~1 &&
+	git show -s --pretty=format:%s HEAD > actual &&
+	test_cmp actual expected
+'
+
+test_debug 'gitk --all'
+
 test_done