diff: widen `deflate_it()`'s bound local from int to `size_t`

Fixes a pre-existing silent narrowing from `git_deflate_bound()`'s
`unsigned long` return into an `int` local: anything past 2 GiB has
always wrapped negative here and then been re-extended to `size_t`
inside `xmalloc()`. Also prep for the upcoming `git_deflate_bound()`
widening to `size_t`, which would extend the narrowing further if
`bound` stayed `int`.

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

diff.c

@@ -3609,7 +3609,7 @@ static unsigned char *deflate_it(char *data,
 				 unsigned long size,
 				 unsigned long *result_size)
 {
-	int bound;
+	size_t bound;
 	unsigned char *deflated;
 	git_zstream stream;
 	struct repo_config_values *cfg = repo_config_values(the_repository);