merge-ort: handle directory/file conflicts that remain

merge-ort: handle directory/file conflicts that remain

When a directory/file conflict remains, we can leave the directory where
it is, but need to move the information about the file to a different
pathname.  After moving the file to a different pathname, we allow
subsequent process_entry() logic to handle any additional details that
might be relevant.

This depends on a new helper function, unique_path(), that dies with an
unimplemented error currently but will be implemented in a subsequent
commit.

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