merge-ort: handle D/F conflict where directory disappears due to merge

merge-ort: handle D/F conflict where directory disappears due to merge

When one side has a directory at a given path and the other side of
history has a file at the path, but the merge resolves the directory
away (e.g. because no path within that directory was modified and the
other side deleted it, or because renaming moved all the files
elsewhere), then we don't actually have a conflict anymore.  We just
need to clear away any information related to the relevant directory,
and then the subsequent process_entry() handling can handle the given
path.

Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>