fix up Read instance incompatability caused by recent commit

9c4650358c changed the Read instance for
Key.

I've checked all uses of that instance (by removing it and seeing what
breaks), and they're all limited to the webapp, except one.
That is GitAnnexDistribution's Read instance.

So, 9c4650358c would have broken upgrades
of git-annex from downloads.kitenet.net. Once the .info files there got
updated for a new release, old releases would have failed to parse them
and never upgraded.

To fix this, I found a way to make the .info files that contain
GitAnnexDistribution values be readable by the old version of git-annex.

This commit was sponsored by Ewen McNeill.

Build/DistributionUpdate.hs

@@ -119,13 +119,14 @@ makeinfos updated version = do
 			Just k -> whenM (inAnnex k) $ do
 				liftIO $ putStrLn f
 				let infofile = f ++ ".info"
-				liftIO $ writeFile infofile $ show $ GitAnnexDistribution
+				let d = GitAnnexDistribution
 					{ distributionUrl = mkUrl f
 					, distributionKey = k
 					, distributionVersion = bv
 					, distributionReleasedate = now
 					, distributionUrgentUpgrade = Nothing
 					}
+				liftIO $ writeFile infofile $ formatInfoFile d
 				void $ inRepo $ runBool [Param "add", File infofile]
 				signFile infofile
 				signFile f