Merge branch 'jk/core-use-replace-refs'

A new configuration variable core.usereplacerefs has been added,
primarily to help server installations that want to ignore the
replace mechanism altogether.

* jk/core-use-replace-refs:
  add core.usereplacerefs config option
  check_replace_refs: rename to read_replace_refs
  check_replace_refs: fix outdated comment

builtin/unpack-objects.c

@@ -514,7 +514,7 @@ int cmd_unpack_objects(int argc, const char **argv, const char *prefix)
 	int i;
 	struct object_id oid;
 
-	check_replace_refs = 0;
+	read_replace_refs = 0;
 
 	git_config(git_default_config, NULL);