This is part of a patch series to extract the memory pool logic in
fast-import into a more generalized version. The existing mem_pool type
maps more closely to a "block of memory" (mp_block) in the more
generalized memory pool. This commit renames the mem_pool to mp_block to
reduce churn in future patches.
Signed-off-by: Jameson Miller <jamill@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>