merge-recursive: Fix code checking for D/F conflicts still being present

merge-recursive: Fix code checking for D/F conflicts still being present

Previously, we were using lstat() to determine if a directory was still
present after a merge (and thus in the way of adding a file).  We should
have been using lstat() only to determine if untracked directories were in
the way (and then only when necessary to check for untracked directories);
we should instead using the index to determine if there is a tracked
directory in the way.  Create a new function to do this and use it to
replace the existing checks for directories being in the way.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>