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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

t/helper/test-mergesort.c

@@ -25,6 +25,7 @@ static int sort_stdin(void)
 	struct line *lines;
 	struct line **tail = &lines;
 	struct strbuf sb = STRBUF_INIT;
+	struct mem_pool lines_pool;
 	char *p;
 
 	strbuf_read(&sb, 0, 0);
@@ -36,10 +37,11 @@ static int sort_stdin(void)
 	if (sb.len && sb.buf[sb.len - 1] == '\n')
 		strbuf_setlen(&sb, sb.len - 1);
 
+	mem_pool_init(&lines_pool, 0);
 	p = sb.buf;
 	for (;;) {
 		char *eol = strchr(p, '\n');
-		struct line *line = xmalloc(sizeof(*line));
+		struct line *line = mem_pool_alloc(&lines_pool, sizeof(*line));
 		line->text = p;
 		*tail = line;
 		tail = &line->next;