Increase the number of possible heads requested from git-upload-pack

Now that git-clone-pack exists, we actually have somebody requesting
more than just a single head in a pack.  So allow the Jeff's of this
world to clone things with tens of heads.

upload-pack.c

@@ -5,7 +5,7 @@
 static const char upload_pack_usage[] = "git-upload-pack <dir>";
 
 #define MAX_HAS (16)
-#define MAX_NEEDS (16)
+#define MAX_NEEDS (256)
 static int nr_has = 0, nr_needs = 0;
 static unsigned char has_sha1[MAX_HAS][20];
 static unsigned char needs_sha1[MAX_NEEDS][20];