reduce delta head inflated size

Supposing that both the base and result sizes were both full size 64-bit
values, their encoding would occupy only 9.2 bytes each.  Therefore
inflating 64 bytes is way overkill.  Limit it to 20 bytes instead which
should be plenty enough for a couple years to come.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

sha1_file.c

@@ -908,7 +908,7 @@ static int packed_delta_info(struct packed_git *p,
 
 	if (sizep) {
 		const unsigned char *data;
-		unsigned char delta_head[64];
+		unsigned char delta_head[20];
 		unsigned long result_size;
 		z_stream stream;
 		int st;