[io] only skip previously seen mergeable TObjects
[io] only skip previously seen mergeable TObjects

Unmergeable TObjects should be passed through unchanged to the output
file. We can't skip them with a continue statement.

olifre/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root