diff-delta: widen sizeof_delta_index() return to size_t

Last piece of the delta API to still expose unsigned long. The
function literally returns struct delta_index.memsize, which became
size_t in the first commit of this series. The sole caller
(free_unpacked() in builtin/pack-objects.c) already accepts size_t
via its freed_mem local, so the widening only removes the implicit
size_t -> unsigned long narrowing inside the function body.

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

delta.h

@@ -28,7 +28,7 @@ void free_delta_index(struct delta_index *index);
  *
  * Given pointer must be what create_delta_index() returned, or NULL.
  */
-unsigned long sizeof_delta_index(struct delta_index *index);
+size_t sizeof_delta_index(struct delta_index *index);
 
 /*
  * create_delta: create a delta from given index for the given buffer

diff-delta.c

@@ -302,7 +302,7 @@ void free_delta_index(struct delta_index *index)
 	free(index);
 }
 
-unsigned long sizeof_delta_index(struct delta_index *index)
+size_t sizeof_delta_index(struct delta_index *index)
 {
 	if (index)
 		return index->memsize;