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>

lib/diff.c

@@ -3720,7 +3720,7 @@ 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);