made parentDir return a Maybe FilePath; removed most uses of it

parentDir is less safe than takeDirectory, especially when working
with relative FilePaths. It's really only useful in loops that
want to terminate at /

This commit was sponsored by Audric SCHILTKNECHT.

Command/ImportFeed.hs

@@ -311,7 +311,7 @@ checkFeedBroken' url f = do
 	now <- liftIO getCurrentTime
 	case prev of
 		Nothing -> do
-			createAnnexDirectory (parentDir f)
+			createAnnexDirectory (takeDirectory f)
 			liftIO $ writeFile f $ show now
 			return False
 		Just prevtime -> do