ByteString Ref continued

Attoparsec parser for diff-tree.

Changed fromRef back to producing a String, to avoid needing to convert
every use of it. However, this does mean I'm going to miss some
opportunities where fromRef is used and the result converted back to a
ByteString. Would be worth revisiting that at some point maybe.

Git.hs

@@ -14,6 +14,7 @@ module Git (
 	Repo(..),
 	Ref(..),
 	fromRef,
+	fromRef',
 	Branch,
 	Sha,
 	Tag,