Remove short-circuitying grubbing by using last grubbed buffer. It's

Remove short-circuitying grubbing by using last grubbed buffer.  It's
evil - if the last grubbed buffer didn't happen to be the right one,
you couldn't remedy.

Mainline emacs compat - don't use third arg to buffer-substring (which
was for explicitly identifying the buffer in which to seek the
substring, and which turns out to be unnecessary).