mem-pool: use consistent pool variable name

mem-pool: use consistent pool variable name

About half the function declarations in mem-pool.h used 'struct mem_pool
*pool', while the other half used 'struct mem_pool *mem_pool'.  Make the
code a bit more consistent by just using 'pool' in preference to
'mem_pool' everywhere.

No behavioral changes included; this is just a mechanical rename (though
a line or two was rewrapped as well).

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