FileDiff: fix file descriptor leak (#885)

When a FileDiff tab starts it creates 6 pipes via CachedSequenceMatcher.
They are never released. Eventually the process runs out of file
descriptors (typically 1024 on linux).

Introduced in commit 8ed5bef Sat Mar 20 09:37:50 2021

     filediff, matchers.helpers: Improve matcher process clean up (#565)

meld/filediff.py

@@ -1303,6 +1303,7 @@ def on_delete_event(self):
 
             try:
                 self._cached_match.stop()
+                self._cached_match = None
             except Exception:
                 # Ignore any cross-process exceptions that happen when
                 # shutting down our matcher process.