merge-recursive: adjust to loosened "working file clobbered" check

merge-recursive: adjust to loosened "working file clobbered" check

The three-way merge by git-read-tree does not complain about
presense of the file in the working tree that is involved in a
merge when the merge result needs to be determined by the
caller.  Adjust merge-recursive so that it makes sure that an
untracked file is not touched when the merge decides the path
should not be included in the final result.

Signed-off-by: Junio C Hamano <junkio@cox.net>