bug#55620: 29.0.50; Eshell doesn't reset `eshell-in-pipeline-p' in subcommands

[Jim Porter <jporterbugs@gmail.com>]

 From `emacs -Q -f eshell':

   ~ $ echo $eshell-in-pipeline-p
   ;; no output
   ~ $ echo ${echo $eshell-in-pipeline-p}
   ;; no output
   ~ $ echo ${echo $eshell-in-pipeline-p} | cat
   first

(Note: in the first two cases, `eshell-in-pipeline-p' is nil, so there's 
no output.) The last case is surprising: it's true that the inner `echo' 
invocation is in a pipeline, but the pipeline is a level above it 
(`first' means that when `eshell-in-pipeline-p' was evaluated, it was 
the first command in a pipeline). Since the inner `echo' is effectively 
in a subshell, it makes more sense for the value of 
`eshell-in-pipeline-p' to be nil for the last case.

While this is a seemingly-obscure issue, there is one practical effect 
that might bite users: currently, the Eshell implementation of `cat' 
only works when *not* in a pipeline. Usually, it'll just fall back to 
using the real system `cat' program if the Eshell version doesn't work, 
but on systems without `cat' (e.g. MS Windows), you get the following error:

   ~ $ echo ${cat file.txt} | echo
   Eshell’s ‘cat’ does not work in pipelines

Patch forthcoming momentarily; just getting a bug number...