environment: add global variable to disable replacement

This new "read_replace_refs" global variable is set to 1 by
default, so that replace refs are used by default. But
reachability traversal and packing commands ("cmd_fsck",
"cmd_prune", "cmd_pack_objects", "upload_pack",
"cmd_unpack_objects") set it to 0, as they must work with the
original DAG.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

replace_object.c

@@ -94,6 +94,9 @@ const unsigned char *lookup_replace_object(const unsigned char *sha1)
 	int pos, depth = MAXREPLACEDEPTH;
 	const unsigned char *cur = sha1;
 
+	if (!read_replace_refs)
+		return sha1;
+
 	prepare_replace_object();
 
 	/* Try to recursively replace the object */