add support for using hashDirLower in addition to hashDirMixed

Supporting multiple directory hash types will allow converting to a
different one, without a flag day.

gitAnnexLocation now checks which of the possible locations have a file.
This means more statting of files. Several places currently use
gitAnnexLocation and immediately check if the returned file exists;
those need to be optimised.

Command/SendKey.hs

@@ -21,7 +21,7 @@ seek = [withKeys start]
 
 start :: Key -> CommandStart
 start key = do
-	file <- fromRepo $ gitAnnexLocation key
+	file <- inRepo $ gitAnnexLocation key
 	whenM (inAnnex key) $
 		liftIO $ rsyncServerSend file -- does not return
 	warning "requested key is not present"