ref-filter: make parse_ref_filter_atom a private function

ref-filter: make parse_ref_filter_atom a private function

The parse_ref_filter_atom() function really shouldn't be
exposed outside of ref-filter.c; its return value is an
integer index into an array that is private in that file.

Since the previous commit removed the sole external caller
(and replaced it with a public function at a more
appropriately level), we can just make this static.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>