run out of tree merge with --no-ff

This is how direct mode does it too, and somehow, for reasons that
currently escape me, this makes git merge not care if it's run with an
empty work tree.

Annex/Direct.hs

@@ -204,6 +204,7 @@ stageMerge d branch commitmode = do
 	-- has been updated, which would leave things in an inconsistent
 	-- state if mergeDirectCleanup is interrupted.
 	-- <http://marc.info/?l=git&m=140262402204212&w=2>
+	liftIO $ print ("stagemerge in", d)
 	merger <- ifM (coreSymlinks <$> Annex.getGitConfig)
 		( return Git.Merge.stageMerge
 		, return $ \ref -> Git.Merge.mergeNonInteractive ref commitmode