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-prune.c

@@ -140,6 +140,7 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 	char *s;
 
 	save_commit_buffer = 0;
+	read_replace_refs = 0;
 	init_revisions(&revs, prefix);
 
 	argc = parse_options(argc, argv, prefix, options, prune_usage, 0);