t/helper/test-pack-deltas: widen do_compress()'s maxsize local to size_t

Prep for the upcoming `git_deflate_bound()` widening to `size_t`. The
local is only ever the return value of `git_deflate_bound()` and the
`xmalloc()`/`stream.avail_out` sizes derived from it; widening it has no
semantic effect today.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-pack-deltas.c

@@ -22,7 +22,7 @@ static unsigned long do_compress(void **pptr, unsigned long size)
 {
 	git_zstream stream;
 	void *in, *out;
-	unsigned long maxsize;
+	size_t maxsize;
 
 	git_deflate_init(&stream, 1);
 	maxsize = git_deflate_bound(&stream, size);