merge-ort: set up a memory pool

merge-ort: set up a memory pool

merge-ort has a lot of data structures, and they all tend to be freed
together in clear_or_reinit_internal_opts().  Set up a memory pool to
allow us to make these allocations and deallocations faster.  Future
commits will adjust various callers to make use of this memory pool.

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