Fix test suite to reap zombies.

I had not taken into account that the code was written to run git and leave
zombies, for performance/laziness reasons, when I wrote the test suite.
So rather than the typical 1 zombie process that git-annex develops, test
developed dozens. Caused problems on system with low process limits.
Added a reap function to GitRepo, that waits for any zombie child processes.

CmdLine.hs

@@ -101,4 +101,7 @@ shutdown = do
 	unless (q == GitQueue.empty) $ do
 		showSideAction "Recording state in git..."
 		Annex.queueRun
+
+	liftIO $ Git.reap
+	
 	return True