merge-recursive: New data structures for deferring of D/F conflicts

merge-recursive: New data structures for deferring of D/F conflicts

Since we need to resolve paths (including renames) in-core first and defer
checking of D/F conflicts (namely waiting to see if directories are still
in the way after all paths are resolved) before updating files involved in
D/F conflicts, we will need to first process_renames, then record some
information about the rename needed at D/F resolution time, and then make
use of that information when resolving D/F conflicts at the end.

This commit adds some relevant data structures for storing the necessary
information.

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