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/Migrate.hs

@@ -49,7 +49,7 @@ upgradableKey key = isNothing $ Types.Key.keySize key
  -}
 perform :: FilePath -> Key -> Backend Annex -> CommandPerform
 perform file oldkey newbackend = do
-	src <- fromRepo $ gitAnnexLocation oldkey
+	src <- inRepo $ gitAnnexLocation oldkey
 	tmp <- fromRepo gitAnnexTmpDir
 	let tmpfile = tmp </> takeFileName file
 	cleantmp tmpfile