diffcore-rename, merge-ort: add wrapper functions for filepair alloc/dealloc

diffcore-rename, merge-ort: add wrapper functions for filepair alloc/dealloc

We want to be able to allocate filespecs and filepairs using a mem_pool.
However, filespec data will still remain outside the pool (perhaps in
the future we could plumb the pool through the various diff APIs to
allocate the filespec data too, but for now we are limiting the scope).
Add some extra functions to allocate these appropriately based on the
non-NULL-ness of opt->priv->pool, as well as some extra functions to
handle correctly deallocating the relevant parts of them.  A future
commit will make use of these new functions.

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