Add a reminder test case for a merge with F/D transition

The problem is that if a file was replaced with a directory containing
another file with the same content and mode, an attempt to merge it
with a branch descended from a commit before this F->D transition will
cause merge-recursive to break. It breaks even if there were no
conflicting changes on that other branch.

Originally reported by Anders Melchiorsen.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6020-merge-df.sh

@@ -22,4 +22,27 @@ git commit -m "File: dir"'
 
 test_expect_code 1 'Merge with d/f conflicts' 'git merge "merge msg" B master'
 
+test_expect_failure 'F/D conflict' '
+	git reset --hard &&
+	git checkout master &&
+	rm .git/index &&
+
+	mkdir before &&
+	echo FILE >before/one &&
+	echo FILE >after &&
+	git add . &&
+	git commit -m first &&
+
+	rm -f after &&
+	git mv before after &&
+	git commit -m move &&
+
+	git checkout -b para HEAD^ &&
+	echo COMPLETELY ANOTHER FILE >another &&
+	git add . &&
+	git commit -m para &&
+
+	git merge master
+'
+
 test_done