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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>