improved git-annex branch changing

All changes to files in the branch are now made via pure functions that
transform the old file into the new. This will allow adding locking
to prevent read/write races. It also makes the code nicer, and purer.

I noticed a behavior change, really a sort of bug fix. Before,
'git annex untrust foo --trust bar' would change both trust levels
permanantly, now the --trust doesn't get stored.

Upgrade/V2.hs

@@ -87,8 +87,8 @@ inject :: FilePath -> FilePath -> Annex ()
 inject source dest = do
 	g <- Annex.gitRepo
 	new <- liftIO (readFile $ olddir g </> source)
-	prev <- Branch.get dest
-	Branch.change dest $ unlines $ nub $ lines prev ++ lines new
+	Branch.change dest $ \prev -> 
+		unlines $ nub $ lines prev ++ lines new
 	showProgress
 
 logFiles :: FilePath -> Annex [FilePath]