Fix serialization of changelog entries

Issue #1525

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/formats/txt.py

@@ -193,7 +193,7 @@ def save(self):
         """Save underlaying store to disk."""
         for filename in self.get_filenames():
             serializer = TextSerializer(
-                os.path.basename(filename), self.store.units
+                self.store.parsers[filename].filename, self.store.units
             )
             self.save_atomic(filename, serializer)