test-mergesort: use mem_pool for sort input

The previous patch almost halved the number of heap allocations for the
sort subcommand.  Reduce it further by using a mem_pool for the line
objects.

Note that t/perf/run can't be used directly to compare two versions of
test-mergesort because it always runs the helpers from the checked-out
version.  So I hand-merged the results of separate runs before and with
this patch:

macOS 12.5.1 on M1:
0071.12: DEFINE_LIST_SORT unsorted     0.22(0.20+0.01)     0.21(0.19+0.01)
0071.14: DEFINE_LIST_SORT sorted       0.10(0.08+0.01)     0.10(0.08+0.01)
0071.16: DEFINE_LIST_SORT reversed     0.10(0.08+0.01)     0.10(0.08+0.01)

Git SDK 64-bit on Windows 11 21H2 on Ryzen 7 5800H:
0071.12: DEFINE_LIST_SORT unsorted     0.54(0.00+0.06)     0.44(0.01+0.06)
0071.14: DEFINE_LIST_SORT sorted       0.21(0.03+0.03)     0.19(0.04+0.01)
0071.16: DEFINE_LIST_SORT reversed     0.21(0.01+0.04)     0.19(0.04+0.04)

Debian bullseye on WSL2 on the same system:
0071.12: DEFINE_LIST_SORT unsorted     0.29(0.27+0.01)     0.22(0.19+0.02)
0071.14: DEFINE_LIST_SORT sorted       0.07(0.06+0.01)     0.06(0.04+0.02)
0071.16: DEFINE_LIST_SORT reversed     0.07(0.04+0.03)     0.06(0.04+0.02)

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
