We stopped using the "prefix" parameter of
relocate_single_git_dir_into_superproject() and its callers in 202275b96b (submodule.c: get_super_prefix_or_empty, 2017-03-14), where
we switched to using the environment global directly.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>