bug#27405: 25.2; Make eshell-next-prompt more reliable

[Pierre Neidhardt]

> >     (defun eshell-previous-prompt (n)
> >       "Move to end of Nth previous prompt in the buffer.
> >     See `eshell-prompt-regexp'."
> >       (interactive "p")
> >       (backward-char)
> >       (eshell-next-prompt (- n))))
>
> What's the backward-char for?

That would have definitely deserved a comment...

Without it, if point is right after the prompt, it won't move. That would be
disturbing for the user.

Detailed rules:

- If point is on the command line _after_ eshell-bol, then go back to 
eshell-bol.

- If point is between eshell-bol (included) and _after_ eshell-bol of the last
prompt (excluded), go to the last prompt.

--
Pierre Neidhardt