add forget command

Works, more or less. --dead is not implemented, and so far a new branch
is made, but keys no longer present anywhere are not scrubbed.

git annex sync fails to push the synced/git-annex branch after a forget,
because it's not a fast-forward of the existing synced branch. Could be
fixed by making git-annex sync use assistant-style sync branches.

Annex/Branch.hs

@@ -402,6 +402,7 @@ handleTransitions :: Transitions -> [Git.Ref] -> Annex (Maybe (Git.Branch, [Git.
 handleTransitions localts refs = do
 	m <- M.fromList <$> mapM getreftransition refs
 	let remotets = M.elems m
+	liftIO $ print ("transitions", localts, remotets)
 	if all (localts ==) remotets
 		then return Nothing
 		else do