bug#59956: 29.0.60: Failure when completing arguments in Eshell after variable interpolation

[Jim Porter]

X-Debbugs-CC: arstoffel@gmail.com

Starting from "emacs -Q -f -eshell", type "echo $exec-path " (note the 
trailing space), and then hit TAB. The result is this error:

  pcomplete-match: Wrong type argument: stringp, ("/usr/bin" ...)

This is a regression from Emacs 28, and it looks like it's due to 
'pcomplete-here-using-help' assuming that all the pcomplete args are 
strings. However, 'exec-path' is a list (and Eshell reports it this way 
to pcomplete), so the completion fails. I think all that's necessary is 
checking that the pcomplete args are strings in 
'pcomplete-here-using-help', but I know next to nothing about pcomplete...