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

Build/DesktopFile.hs

@@ -6,6 +6,7 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
+{-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE CPP #-}
 {-# OPTIONS_GHC -fno-warn-tabs #-}