function commitPicker(picker) {
        var filter = getActiveFilter();
        return (filter && filter.patterns.length) ? compile(filter.patterns) : "";
    }
Marks the filter picker's currently selected item as most-recently used, and returns the corresponding
'compiled' filter object ready for use with filterPath().
@param {!jQueryObject} picker UI returned from createFilterPicker()
@return {!string} 'compiled' filter that can be passed to filterPath()/filterFileList().