mem-pool: fix big allocations

mem-pool: fix big allocations

Memory pool allocations that require a new block and would fill at
least half of it are handled specially.  Before 158dfeff3d (mem-pool:
add life cycle management functions, 2018-07-02) they used to be
allocated outside of the pool.  This patch made mem_pool_alloc() create
a bespoke block instead, to allow releasing it when the pool gets
discarded.

Unfortunately mem_pool_alloc() returns a pointer to the start of such a
bespoke block, i.e. to the struct mp_block at its top.  When the caller
writes to it, the management information gets corrupted.  This affects
mem_pool_discard() and -- if there are no other blocks in the pool --
also mem_pool_alloc().

Return the payload pointer of bespoke blocks, just like for smaller
allocations, to protect the management struct.

Also update next_free to mark the block as full.  This is only strictly
necessary for the first allocated block, because subsequent ones are
inserted after the current block and never considered for further
allocations, but it's easier to just do it in all cases.

Add a basic unit test to demonstrate the issue by using
mem_pool_calloc() with a tiny block size, which forces the creation of a
bespoke block.

Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>