bug#46351: 28.0.50; Add convenient way to bypass Eshell's own pipelining

[Eli Zaretskii]

> From: Sean Whitton <spwhitton@spwhitton.name>
> Cc: michael.albinus@gmx.de, 46351@debbugs.gnu.org
> Date: Tue, 18 Jan 2022 11:40:50 -0700
> 
> > I see you play some games with shell syntactic rules and such: IME,
> > these are often tricky enough to defeat simplistic handling.  For
> > example, does this support commands that have redirection operators
> > _before_ the first verb?  It's allowed in every shell I've seen,
> > albeit very rarely used.
> 
> These things are indeed tricky.  So long as it is not too easy to
> activate the functionality by accident, however, I think that it will be
> very helpful with regard to one of the most common reasons people cite
> for not using Eshell very much.

OK, but I don't think I see how this answers my question.  Is it
possible today to say

  >foo something or other

in Eshell, and have the output of 'something' redirected to 'foo'?  If
it is possible today, will it be possible after your changes, and what
will happen with that if 'something' includes the new "*|" pipe
symbol?

> > Why do these tests only look for 'sh' as the shell?  What is the
> > importance of the shell for this purpose?
> 
> The code just substitutes in the value of `shell-file-name' (taking
> TRAMP into account), so there isn't much point in varying that value in
> the tests, so I just picked something standard.  The user is expected to
> know what syntax their external shell will accept and what it will do
> with it when they invoke this functionality.

Does this mean that those tests will not run on systems where 'sh' is
not available?