refs.h: implement hidden_refs_to_excludes()

In subsequent commits, we'll teach `receive-pack` and `upload-pack` to
use the new jump list feature in the packed-refs iterator by ignoring
references which are mentioned via its respective hideRefs lists.

However, the packed-ref jump lists cannot handle un-hiding rules (that
begin with '!'), or namespace comparisons (that begin with '^'). Add a
convenience function to the refs.h API to detect when either of these
conditions are met, and returns an appropriate value to pass as excluded
patterns.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -831,6 +831,12 @@ int parse_hide_refs_config(const char *var, const char *value, const char *,
  */
 int ref_is_hidden(const char *, const char *, const struct strvec *);
 
+/*
+ * Returns an array of patterns to use as excluded_patterns, if none of the
+ * hidden references use the token '!' or '^'.
+ */
+const char **hidden_refs_to_excludes(const struct strvec *hide_refs);
+
 /* Is this a per-worktree ref living in the refs/ namespace? */
 int is_per_worktree_ref(const char *refname);