repository: create disable_replace_refs()

Several builtins depend on being able to disable the replace references
so we actually operate on each object individually. These currently do
so by directly mutating the 'read_replace_refs' global.

A future change will move this global into a different place, so it will
be necessary to change all of these lines. However, we can simplify that
transition by abstracting the purpose of these global assignments with a
method call.

We will need to keep this read_replace_refs global forever, as we want
to make sure that we never use replace refs throughout the life of the
process if this method is called. Future changes may present a
repository-scoped version of the variable to represent that repository's
core.useReplaceRefs config value, but a zero-valued read_replace_refs
will always override such a setting.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

replace-object.c

@@ -84,3 +84,8 @@ const struct object_id *do_lookup_replace_object(struct repository *r,
 	}
 	die(_("replace depth too high for object %s"), oid_to_hex(oid));
 }
+
+void disable_replace_refs(void)
+{
+	read_replace_refs = 0;
+}