The mercurial backend is broken when no changelog message is given.

Here is a quick patch, partialy copying the svn backend. 

    --- /usr/share/perl5/IkiWiki/Rcs/mercurial.pm	2007-03-18 23:19:40.000000000 +0100
    +++ ./mercurial.pm	2007-03-24 13:11:36.000000000 +0100
    @@ -70,12 +70,15 @@
     
     	if (defined $user) {
     		$user = possibly_foolish_untaint($user);
    +		$message="web commit by $user".(length $message ? ": $message" : "");
     	}
     	elsif (defined $ipaddr) {
     		$user = "Anonymous from $ipaddr";
    +		$message="web commit from $ipaddr".(length $message ? ": $message" : "");
     	}
     	else {
     		$user = "Anonymous";
    +		$message="web commit by Anonymous".(length $message ? ": $message" : "");
     	}
     
     	$message = possibly_foolish_untaint($message);