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/RefLog.hs

@@ -18,5 +18,5 @@ get b = mapMaybe extractSha . lines <$$> pipeReadStrict
 	[ Param "log"
 	, Param "-g"
 	, Param "--format=%H"
-	, Param (show b)
+	, Param (fromRef b)
 	]