Make notebook tab-completion in R evaluation work right

[kcrisman]

From ​this stackoverflow post and ​this ask.sagemath post, the same question:

I know that when calls R with Sage,the dot will convert to underscores.
But even when I set the "system" to R with combo box above in notebook,
the dot will convert to underscores, and that is the problem. 
Is there any solution?

This is now ​Github issue 103. Note that I'm not sure whether this is in Sage or sagenb, though sagenb seems plausible.