remove Read instance for Ref

Removed instance, got it all to build using fromRef. (With a few things
that really need to show something using a ref for debugging stubbed out.)

Then added back Read instance, and made Logs.View use it for serialization.
This changes the view log format.

Git.hs

@@ -13,6 +13,7 @@
 module Git (
 	Repo(..),
 	Ref(..),
+	fromRef,
 	Branch,
 	Sha,
 	Tag,