Add clone hints to 404 error pages

When clicking on the linked Git clone URL of a package base, users are
faced with a 404 error page since the URL is not supposed to be opened
in a web browser. Add some notes to 404 error pages corresponding to Git
clone URLs that explain how to use them instead.

Fixes FS#51266.
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

web/html/404.php

@@ -2,12 +2,45 @@
 
 set_include_path(get_include_path() . PATH_SEPARATOR . '../lib');
 
+$path = $_SERVER['PATH_INFO'];
+$tokens = explode('/', $path);
+
+if (preg_match('/^([a-z0-9][a-z0-9.+_-]*?)(\.git)?$/', $tokens[1], $matches)) {
+	$gitpkg = $matches[1];
+	if (pkg_from_name($gitpkg)) {
+		$gitcmd = 'git clone ' . sprintf(config_get('options', 'git_clone_uri_anon'), htmlspecialchars($gitpkg));
+		$gitlink = get_pkgbase_uri($gitpkg);
+	} else {
+		unset($gitpkg);
+	}
+} else {
+	unset($gitpkg);
+}
+
 html_header( __("Page Not Found") );
 ?>
 
 <div id="error-page" class="box 404">
 	<h2>404 - <?= __("Page Not Found") ?></h2>
 	<p><?= __("Sorry, the page you've requested does not exist.") ?></p>
+	<?php if (isset($gitpkg)): ?>
+	<ul>
+		<li>
+			<strong><?= __("Note") ?>:</strong>
+			<?= __("Git clone URLs are not meant to be opened in a browser.") ?>
+		</li>
+		<li>
+			<?= __("To clone the Git repository of %s, run %s.",
+				'<strong>' . htmlspecialchars($gitpkg) . '</strong>',
+				'<code>' . htmlspecialchars($gitcmd) . '</code>') ?>
+		</li>
+		<li>
+			<?= __("Click %shere%s to return to the %s details page.",
+				'<a href="' . htmlspecialchars($gitlink, ENT_QUOTES) . '">', '</a>',
+				'<strong>' . htmlspecialchars($gitpkg) . '</strong>') ?>
+		</li>
+	</ul>
+	<?php endif; ?>
 </div>
 
 <?php