bug#60464: 29.0.60; Regression - pcomplete-arg fails with argument 'last

[Gregory Heytings]

> > Feel free to suggest something else.
>
> Yes, I did. The command line string should be returned.

I mean, to propose some other code.

> I believe you that doing this correctly is non trivial. But this doesn't 
> justify going with your hack.

It's extracting the correct part of the command line string that is hacky, 
too hacky to my taste.