modify/delete conflict resolution overwrites untracked file

modify/delete conflict resolution overwrites untracked file

If a file was removed in HEAD, but modified in MERGE_HEAD, recursive merge
will result in a "CONFLICT (delete/modify)". If the (now untracked) file
already exists and was not added to the index, it is overwritten with the
conflict resolution contents.

In similar situations (cf. test 2), the merge would abort with

"error: Untracked working tree 'file' would be overwritten by merge."

The same should happen in this case.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>