RepositoryUtils: fix GitLab URL generation

GitLab started using a `/-/` component to act as a separator between
project paths and project resources.

app/Utils/RepositoryUtils.php

@@ -449,8 +449,20 @@ public static function get_github_diff_url($projecturl, $directory, $file, $revi
     /** Return the GitLab diff URL */
     public static function get_gitlab_diff_url($projecturl, $directory, $file, $revision)
     {
-        // GitLab uses 'blob' or 'tree' (singular, no s)
-        return self::get_gitoriousish_diff_url($projecturl, $directory, $file, $revision, 'blob');
+        // Since GitLab supports arbitrarily nested groups, there is a `/-/`
+        // component to start per-project resources.
+        if ($revision != '') {
+            $diff_url = $projecturl . '/-/commit/' . $revision;
+        } elseif ($file != '') {
+            $diff_url = $projecturl . '/-/blob/' . $branch . '/';
+            if ($directory != '') {
+                $diff_url .= $directory . '/';
+            }
+            $diff_url .= $file;
+        } else {
+            return '';
+        }
+        return make_cdash_url($diff_url);
     }
 
     /** Return the cgit diff URL */