merge-ort: add handling for different types of files at same path

merge-ort: add handling for different types of files at same path

Add some handling that explicitly considers collisions of the following
types:
  * file/submodule
  * file/symlink
  * submodule/symlink
Leaving them as conflicts at the same path are hard for users to
resolve, so move one or both of them aside so that they each get their
own path.

Note that in the case of recursive handling (i.e. call_depth > 0), we
can just use the merge base of the two merge bases as the merge result
much like we do with modify/delete conflicts, binary files, conflicting
submodule values, and so on.

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