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 <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -364,7 +364,7 @@ struct atom_value {
 /*
  * Used to parse format string and sort specifiers
  */
-int parse_ref_filter_atom(const char *atom, const char *ep)
+static int parse_ref_filter_atom(const char *atom, const char *ep)
 {
 	const char *sp;
 	const char *arg;