Fix detection of parent-relative paths in playlist saving

File or directory names containing ``..`` caused saved playlists to
contain absolute paths instead of relative paths even when the relative
path was "safe".  This changes the detection to be more strict.

Potentially related to #4463.

src/playlistparsers/parserbase.cpp

@@ -91,7 +91,7 @@ QString ParserBase::URLOrRelativeFilename(const QUrl& url,
   if (QDir::isAbsolutePath(filename)) {
     const QString relative = dir.relativeFilePath(filename);
 
-    if (!relative.contains("..")) return relative;
+    if (!relative.startsWith("../")) return relative;
   }
   return filename;
 }