optimise literalOsPath

Taking a ShortByteString and using OverloadedStrings should avoid it
being converted from a String.

The reason there is no IsString instance for OsPath is presumably the
bad behavior of IsString for ByteString on unicode btw. But
literalOsPath won't be used with unicode in git-annex.

Sponsored-by: unqueued

Utility/SshConfig.hs

@@ -5,6 +5,8 @@
  - License: BSD-2-clause
  -}
 
+{-# LANGUAGE OverloadedStrings #-}
+
 module Utility.SshConfig (
 	SshConfig(..),
 	Comment(..),