test-mergesort: read sort input all at once

test-mergesort: read sort input all at once

The sort subcommand of test-mergesort is used to test the performance of
sorting linked lists.  It reads lines from stdin, sorts them and prints
the result to stdout.  Two heap allocations are done per line: One for
the linked list item and one for the actual line string.  That imposes a
significant amount of allocation overhead.

Reduce it by doing the same as the sort subcommand of test-string-list,
namely to read the whole input file into a single buffer and then split
it in-place.

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     0.23(0.20+0.01)     0.22(0.20+0.01)
0071.14: DEFINE_LIST_SORT sorted       0.12(0.10+0.01)     0.10(0.08+0.01)
0071.16: DEFINE_LIST_SORT reversed     0.12(0.10+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     0.71(0.00+0.03)     0.54(0.00+0.06)
0071.14: DEFINE_LIST_SORT sorted       0.42(0.00+0.04)     0.21(0.03+0.03)
0071.16: DEFINE_LIST_SORT reversed     0.42(0.06+0.01)     0.21(0.01+0.04)

Debian bullseye on WSL2 on the same system:
0071.12: DEFINE_LIST_SORT unsorted     0.41(0.39+0.02)     0.29(0.27+0.01)
0071.14: DEFINE_LIST_SORT sorted       0.11(0.08+0.02)     0.07(0.06+0.01)
0071.16: DEFINE_LIST_SORT reversed     0.11(0.08+0.02)     0.07(0.04+0.03)

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