merge-recursive: fix overwriting dirty files involved in renames

merge-recursive: fix overwriting dirty files involved in renames

This fixes an issue that existed before my directory rename detection
patches that affects both normal renames and renames implied by
directory rename detection.  Additional codepaths that only affect
overwriting of dirty files that are involved in directory rename
detection will be added in a subsequent commit.

Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>