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.c

@@ -1480,6 +1480,30 @@ int ref_is_hidden(const char *refname, const char *refname_full,
 	return 0;
 }
 
+const char **hidden_refs_to_excludes(const struct strvec *hide_refs)
+{
+	const char **pattern;
+	for (pattern = hide_refs->v; *pattern; pattern++) {
+		/*
+		 * We can't feed any excludes from hidden refs config
+		 * sections, since later rules may override previous
+		 * ones. For example, with rules "refs/foo" and
+		 * "!refs/foo/bar", we should show "refs/foo/bar" (and
+		 * everything underneath it), but the earlier exclusion
+		 * would cause us to skip all of "refs/foo".  We
+		 * likewise don't implement the namespace stripping
+		 * required for '^' rules.
+		 *
+		 * Both are possible to do, but complicated, so avoid
+		 * populating the jump list at all if we see either of
+		 * these patterns.
+		 */
+		if (**pattern == '!' || **pattern == '^')
+			return NULL;
+	}
+	return hide_refs->v;
+}
+
 const char *find_descendant_ref(const char *dirname,
 				const struct string_list *extras,
 				const struct string_list *skip)

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);