Correct max_packsize default in fast-import.

Apparently amd64 has defined 'unsigned long' to be a 64 bit value,
which means -1 was way over the 4 GiB packfile limit.  Whoops.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -217,7 +217,7 @@ struct hash_list
 
 /* Configured limits on output */
 static unsigned long max_depth = 10;
-static unsigned long max_packsize = -1;
+static unsigned long max_packsize = (1LL << 32) - 1;
 static uintmax_t max_objects = -1;
 
 /* Stats and misc. counters */