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/Add.hs

@@ -60,7 +60,7 @@ undo file key e = do
 		-- fromAnnex could fail if the file ownership is weird
 		tryharder :: IOException -> Annex ()
 		tryharder _ = do
-			src <- fromRepo $ gitAnnexLocation key
+			src <- inRepo $ gitAnnexLocation key
 			liftIO $ moveFile src file
 
 cleanup :: FilePath -> Key -> Bool -> CommandCleanup