Changing the output of [gallery] tag (wp-includes/media.php)

I would like to change the output of the [gallery] tag. Specifically, I want to make the ‘caption’ text link to the original picture file.

The relevant chunk of code seems to be this, but I don’t know how to do it. Can somebody help me along?

/**
 * The Gallery shortcode.
 *
 * This implements the functionality of the Gallery Shortcode for displaying
 * WordPress images on a post.
 *
 * @since 2.5.0
 *
 * @param array $attr Attributes attributed to the shortcode.
 * @return string HTML content to display gallery.
 */
function gallery_shortcode($attr) {
	global $post;

	static $instance = 0;
	$instance++;

	// Allow plugins/themes to override the default gallery template.
	$output = apply_filters('post_gallery', '', $attr);
	if ( $output != '' )
		return $output;

	// We're trusting author input, so let's at least make sure it looks like a valid orderby statement
	if ( isset( $attr['orderby'] ) ) {
		$attr['orderby'] = sanitize_sql_orderby( $attr['orderby'] );
		if ( !$attr['orderby'] )
			unset( $attr['orderby'] );
	}

	extract(shortcode_atts(array(
		'order'      => 'ASC',
		'orderby'    => 'menu_order ID',
		'id'         => $post->ID,
		'itemtag'    => 'dl',
		'icontag'    => 'dt',
		'captiontag' => 'dd',
		'columns'    => 3,
		'size'       => 'thumbnail'
	), $attr));

	$id = intval($id);
	$attachments = get_children( array('post_parent' => $id, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );

	if ( empty($attachments) )
		return '';

	if ( is_feed() ) {
		$output = "\n";
		foreach ( $attachments as $att_id => $attachment )
			$output .= wp_get_attachment_link($att_id, $size, true) . "\n";
		return $output;
	}

	$itemtag = tag_escape($itemtag);
	$captiontag = tag_escape($captiontag);
	$columns = intval($columns);
	$itemwidth = $columns > 0 ? floor(100/$columns) : 100;

	$selector = "gallery-{$instance}";

	$output = apply_filters('gallery_style', "
		<style type='text/css'>
			#{$selector} {
				margin: auto;
			}
			#{$selector} .gallery-item {
				float: left;
				margin-top: 10px;
				text-align: center;
				width: {$itemwidth}%;			}
			#{$selector} img {
				border: 2px solid #cfcfcf;
			}
			#{$selector} .gallery-caption {
				margin-left: 0;
			}
		</style>
		<!-- see gallery_shortcode() in wp-includes/media.php -->
		<div id='$selector' class='gallery galleryid-{$id}'>");

	$i = 0;
	foreach ( $attachments as $id => $attachment ) {
		$link = isset($attr['link']) && 'file' == $attr['link'] ? wp_get_attachment_link($id, $size, false, false) : wp_get_attachment_link($id, $size, true, false);

		$output .= "<{$itemtag} class='gallery-item test'>";
		$output .= "
			<{$icontag} class='gallery-icon'>
				$link
			</{$icontag}>";
		if ( $captiontag && trim($attachment->post_excerpt) ) {
			$output .= "
				<{$captiontag} class='gallery-caption'>
				" . wptexturize($attachment->post_excerpt) . "
				</{$captiontag}>";
		}
		$output .= "</{$itemtag}>";
		if ( $columns > 0 && ++$i % $columns == 0 )
			$output .= '<br style="clear: both" />';
	}

	$output .= "
			<br style='clear: both;' />
		</div>\n";

	return $output;
}