t6044: recursive can silently incorporate dirty changes in a merge

The recursive merge strategy has some special handling when the tree for
the merge branch exactly matches the merge base, but that code path is
missing checks for the index having changes relative to HEAD.  Add a
testcase covering this scenario.
Reported-by: NAndreas Krey <a.krey@gmx.de>
Signed-off-by: NElijah Newren <newren@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t6044-merge-unrelated-index-changes.sh

@@ -6,18 +6,21 @@ test_description="merges with unrelated index changes"
 
 # Testcase for some simple merges
 #   A
-#   o-----o B
+#   o-------o B
 #    \
-#     \---o C
+#     \-----o C
 #      \
-#       \-o D
+#       \---o D
 #        \
-#         o E
+#         \-o E
+#          \
+#           o F
 #   Commit A: some file a
 #   Commit B: adds file b, modifies end of a
 #   Commit C: adds file c
 #   Commit D: adds file d, modifies beginning of a
 #   Commit E: renames a->subdir/a, adds subdir/e
+#   Commit F: empty commit
 
 test_expect_success 'setup trivial merges' '
 	test_seq 1 10 >a &&
@@ -29,6 +32,7 @@ test_expect_success 'setup trivial merges' '
 	git branch C &&
 	git branch D &&
 	git branch E &&
+	git branch F &&
 
 	git checkout B &&
 	echo b >b &&
@@ -52,7 +56,10 @@ test_expect_success 'setup trivial merges' '
 	git mv a subdir/a &&
 	echo e >subdir/e &&
 	git add subdir &&
-	test_tick && git commit -m E
+	test_tick && git commit -m E &&
+
+	git checkout F &&
+	test_tick && git commit --allow-empty -m F
 '
 
 test_expect_success 'ff update' '
@@ -105,6 +112,15 @@ test_expect_success 'recursive' '
 	test_must_fail git merge -s recursive C^0
 '
 
+test_expect_failure 'recursive, when merge branch matches merge base' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s recursive F^0
+'
+
 test_expect_success 'octopus, unrelated file touched' '
 	git reset --hard &&
 	git checkout B^0 &&