diff: cleanup the fifos when FIFOFEEDER context manager exits

These files can be seen in thousands, named "fifo1" and "fifo2", under the
temporary directory while diffoscope still runs. Instead of cleaning up
massively in the end, we now clean up while we go.

diffoscope/diff.py

@@ -213,6 +213,7 @@ class FIFOFeeder(threading.Thread):
         return self
 
     def __exit__(self, exc_type, exc_value, exc_tb):
+        os.remove(self.fifo_path)
         self.join()
 
     def run(self):