diff-delta: widen `struct delta_index`' size fields to `size_t`

Preparation for widening the delta-encoding API to `size_t` in
subsequent commits, which is what lets pack-objects drop the
`cast_size_t_to_ulong()` shims that 606c192380 (odb, packfile: use
size_t for streaming object sizes, 2026-05-08) had to leave behind in
`get_delta()` and `try_delta()` because their downstream consumers were
still narrow.

The struct is private to diff-delta.c, so widening its fields in
isolation is a no-op at runtime: the values stored continue to fit in 32
bits on Windows because the public API around it still truncates.
Splitting it out keeps the API-change commit focused on caller updates.

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

diff-delta.c

@@ -125,9 +125,9 @@ struct unpacked_index_entry {
 };
 
 struct delta_index {
-	unsigned long memsize;
+	size_t memsize;
 	const void *src_buf;
-	unsigned long src_size;
+	size_t src_size;
 	unsigned int hash_mask;
 	struct index_entry *hash[FLEX_ARRAY];
 };
@@ -140,7 +140,7 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 	struct unpacked_index_entry *entry, **hash;
 	struct index_entry *packed_entry, **packed_hash;
 	void *mem;
-	unsigned long memsize;
+	size_t memsize;
 
 	if (!buf || !bufsize)
 		return NULL;