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.

Backend/SHA.hs

@@ -99,7 +99,7 @@ keyValueE size file = keyValue size file >>= maybe (return Nothing) addE
 checkKeyChecksum :: SHASize -> Key -> Annex Bool
 checkKeyChecksum size key = do
 	fast <- Annex.getState Annex.fast
-	file <- fromRepo $ gitAnnexLocation key
+	file <- inRepo $ gitAnnexLocation key
 	present <- liftIO $ doesFileExist file
 	if not present || fast
 		then return True