mediawiki.jqueryMsg: Don't duplicate link contents if parse() is called multiple...

mediawiki.jqueryMsg: Don't duplicate link contents if parse() is called multiple times

    mw.messages.set( 'foo', '[$1 Link]' );
    var msg = mw.message( 'foo', $( '<a>' ).attr( 'href', 'http://example.com/' ) );
    msg.parse(); msg.parse(); msg.parse();
    $( 'body ').append( msg.parse() ); // The link now says "LinkLinkLinkLink"

Change-Id: Id170b328164527320326178f24f56c5077cd50ab