merge-ort: add a handle_deferred_entries() helper function

merge-ort: add a handle_deferred_entries() helper function

In order to allow trivial directory resolution, we first need to be able
to gather more information to determine if the optimization is safe.  To
enable that, we need a way of deferring the recursion into the directory
until a later time.  Naturally, deferring the entry into a subtree means
that we need some function that will later recurse into the subdirectory
exactly the same way that collect_merge_info_callback() would have done.

Add a helper function that does this.  For now this function is not used
but a subsequent commit will change that.  Future commits will also make
the function sometimes resolve directories instead of traversing inside.

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