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>

builtin-unpack-objects.c

@@ -495,6 +495,8 @@ int cmd_unpack_objects(int argc, const char **argv, const char *prefix)
 	int i;
 	unsigned char sha1[20];
 
+	read_replace_refs = 0;
+
 	git_config(git_default_config, NULL);
 
 	quiet = !isatty(2);