Improve the safety check used in fetch.c

Improve the safety check used in fetch.c

The recent safety check to trust only the commits we have made
things impossibly slow and turn out to waste a lot of memory.

This commit fixes it with the following improvements:

 - mark already scanned objects and avoid rescanning the same
   object again;

 - free the tree entries when we have scanned the tree entries;
   this is the same as b0d8923ec01fd91b75ab079034f89ced91500157
   which reduced memory usage by rev-list;

 - plug memory leak from the object_list dequeuing code;

 - use the process_queue not just for fetching but for scanning,
   to make things tail recursive to avoid deep recursion; the
   deep recursion was especially prominent when we cloned a big
   pack.

 - avoid has_sha1_file() call when we already know we do not have
   that object.

Signed-off-by: Junio C Hamano <junkio@cox.net>