FileDigester: eliminate _pipe_logger_exit with _async_waitpid

Eliminate the _pipe_logger_exit method by overriding the
_async_waitpid method.

Signed-off-by: Zac Medico <zmedico@gentoo.org>

lib/portage/util/_async/FileDigester.py

@@ -57,10 +57,11 @@ def _parse_digests(self, data):
 
 		self.digests = digests
 
-	def _pipe_logger_exit(self, pipe_logger):
+	def _async_waitpid(self):
 		# Ignore this event, since we want to ensure that we
 		# exit only after _digest_pipe_reader has reached EOF.
-		self._pipe_logger = None
+		if self._digest_pipe_reader is None:
+			ForkProcess._async_waitpid(self)
 
 	def _digest_pipe_reader_exit(self, pipe_reader):
 		self._parse_digests(pipe_reader.getvalue())