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.

Remote/BitTorrent.hs

@@ -189,7 +189,7 @@ downloadTorrentFile u = do
 		, do
 			showAction "downloading torrent file"
 			showOutput
-			createAnnexDirectory (parentDir torrent)
+			createAnnexDirectory (takeDirectory torrent)
 			if isTorrentMagnetUrl u
 				then do
 					tmpdir <- tmpTorrentDir u