merge-ort: avoid repeating fill_tree_descriptor() on the same tree

merge-ort: avoid repeating fill_tree_descriptor() on the same tree

Three-way merges, by their nature, are going to often have two or more
trees match at a given subdirectory.  We can avoid calling
fill_tree_descriptor() on the same tree by checking when these trees
match.  Noting when various oids match will also be useful in other
calculations and optimizations as well.

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