read_index(): fix reading extension size on BE 64-bit archs

On big endian platforms with 8-byte unsigned long, the code reads the
size of the index extension section (which is a 4-byte network byte
order integer) incorrectly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1016,7 +1016,7 @@ int read_index_from(struct index_state *istate, const char *path)
 		 * extension name (4-byte) and section length
 		 * in 4-byte network byte order.
 		 */
-		unsigned long extsize;
+		uint32_t extsize;
 		memcpy(&extsize, (char *)mmap + src_offset + 4, 4);
 		extsize = ntohl(extsize);
 		if (read_index_extension(istate,