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>

upload-pack.c

@@ -618,6 +618,7 @@ int main(int argc, char **argv)
 	int strict = 0;
 
 	git_extract_argv0_path(argv[0]);
+	read_replace_refs = 0;
 
 	for (i = 1; i < argc; i++) {
 		char *arg = argv[i];