refs: parse_hide_refs_config to use parse_config_key

refs: parse_hide_refs_config to use parse_config_key

parse_config_key was introduced in 1b86bbb0ade (config: add helper
function for parsing key names, 2013-01-22), the NEEDSWORK that is removed
in this patch was introduced at daebaa7813 (upload/receive-pack: allow
hiding ref hierarchies, 2013-01-18), which is only a couple days apart,
so presumably the code replaced in this patch was only introduced due
to not wanting to wait on the proper helper function being available.

Make the condition easier to read by using parse_config_key.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>