optimise single dot detection

Since it already looks for things starting with a dot, I was able to avoid
matching against the string twice.

This also fixes a minor bug; $from may not be defined. Avoid uninitialized
value warnings in this case.

IkiWiki.pm

@@ -2401,15 +2401,18 @@ package IkiWiki::PageSpec;
 sub derel ($$) {
 	my $path=shift;
 	my $from=shift;
-	if ($path eq '.') {
-		$path = $from;
-	}
 
-	if ($path =~ m!^\./!) {
+	if ($path =~ m!^\.(/|$)!) {
+		if ($1) {
 			$from=~s#/?[^/]+$## if defined $from;
 			$path=~s#^\./##;
 			$path="$from/$path" if defined $from && length $from;
 		}
+		else {
+			$path = $from;
+			$path = "" unless defined $path;
+		}
+	}
 
 	return $path;
 }