D6655: continue: added support for graft

[Yuya Nishihara]

> It looks like pretty much this entire function is copies `commands.graft()`
> and then all this code is left dead here (since `cont = True` above).
> I'd prefer to de-queue this commit and let you clean this up and then
> submit a new patch

That's my feeling. Perhaps the core `for` loop can be extracted to a helper
function. Or alternatively, `continuegraft()` can be implemented on top of
commands.graft()?