mergetool: support delete/delete conflicts

mergetool: support delete/delete conflicts

If two branches each move a file into different directories then
mergetool will fail because it assumes that the file being merged, and
its parent directory, are present in the worktree.

Create the merge file's parent directory to allow using the
deleted base version of the file for merge resolution when
encountering a delete/delete conflict.

The end result is that a delete/delete conflict is presented for the
user to resolve.

Reported-by: Joe Einertson <joe@kidblog.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>