diffcore: widen struct diff_filespec.size to size_t

Continue the size_t evacuation. The struct field already receives
its writes from a size_t-shaped source (xsize_t(st.st_size),
strbuf.len, fill_textconv()'s return, odb_read_object_info_extended()
via oi.sizep), so on Windows it was already truncating anything
past 4 GiB silently on the strbuf and textconv paths and loudly
through cast_size_t_to_ulong() on the odb path. Switch the field
to size_t.

In diff_populate_filespec(), point oi.sizep at the field directly
and drop both cast_size_t_to_ulong() shims and the size_st bridge
they fed.

Downstream consumers that still read .size into unsigned long
locals will now silently narrow on Windows where the field exceeds
4 GiB. Each of those is its own follow-up; the writer side is the
prerequisite for ever putting a >4 GiB value in the field in the
first place.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

diff.c

@@ -4595,9 +4595,8 @@ int diff_populate_filespec(struct repository *r,
 		}
 	}
 	else {
-		size_t size_st = 0;
 		struct object_info info = {
-			.sizep = &size_st
+			.sizep = &s->size
 		};
 
 		if (!(size_only || check_binary))
@@ -4619,7 +4618,6 @@ int diff_populate_filespec(struct repository *r,
 			die("unable to read %s", oid_to_hex(&s->oid));
 
 object_read:
-		s->size = cast_size_t_to_ulong(size_st);
 		if (size_only || check_binary) {
 			if (size_only)
 				return 0;
@@ -4634,7 +4632,6 @@ object_read:
 			if (odb_read_object_info_extended(r->objects, &s->oid, &info,
 							  OBJECT_INFO_LOOKUP_REPLACE))
 				die("unable to read %s", oid_to_hex(&s->oid));
-			s->size = cast_size_t_to_ulong(size_st);
 		}
 		s->should_free = 1;
 	}

diffcore.h

@@ -54,7 +54,7 @@ struct diff_filespec {
 	char *path;
 	void *data;
 	void *cnt_data;
-	unsigned long size;
+	size_t size;
 	int count;               /* Reference count */
 	int rename_used;         /* Count of rename users */
 	unsigned short mode;	 /* file mode */