gitweb: Add git_get_{following,preceding}_references functions

Adds git_get_following_references function, based on code which was
used in git_commitdiff_plain to generate X-Git-Tag: header,
and companion git_get_preceding_references function.

Both functions return array of all references of given type (as
returned by git_get_references) following/preceding given commit in
array (list) context, and last following/first preceding ref in scalar
context.

Stripping ref (list of refs) of "$type/" (e.g. "tags/") is left to
caller.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.perl

@@ -749,6 +749,58 @@ sub git_get_references {
 	return \%refs;
 }
 
+sub git_get_following_references {
+	my $hash = shift || return undef;
+	my $type = shift;
+	my $base = shift || $hash_base || "HEAD";
+
+	my $refs = git_get_references($type);
+	open my $fd, "-|", $GIT, "rev-list", $base
+		or return undef;
+	my @commits = map { chomp; $_ } <$fd>;
+	close $fd
+		or return undef;
+
+	my @reflist;
+	my $lastref;
+
+	foreach my $commit (@commits) {
+		foreach my $ref (@{$refs->{$commit}}) {
+			$lastref = $ref;
+			push @reflist, $lastref;
+		}
+		if ($commit eq $hash) {
+			last;
+		}
+	}
+
+	return wantarray ? @reflist : $lastref;
+}
+
+sub git_get_preceding_references {
+	my $hash = shift || return undef;
+	my $type = shift;
+
+	my $refs = git_get_references($type);
+	open my $fd, "-|", $GIT, "rev-list", $hash
+		or return undef;
+	my @commits = map { chomp; $_ } <$fd>;
+	close $fd
+		or return undef;
+
+	my @reflist;
+	my $firstref;
+
+	foreach my $commit (@commits) {
+		foreach my $ref (@{$refs->{$commit}}) {
+			$firstref = $ref unless $firstref;
+			push @reflist, $ref;
+		}
+	}
+
+	return wantarray ? @reflist : $firstref;
+}
+
 ## ----------------------------------------------------------------------
 ## parse to hash functions