pcomplete.el: Fix part of bug#50470

pcomplete.el: Fix part of bug#50470

Try and handle the case where `pcomplete-parse-arguments-function`
directly returns a list of completions.

* lisp/pcomplete.el (pcomplete-parse-arguments): Don't modify the
buffer if we're not allowed to.  Instead use the buffer's current
content as the "pattern to be completed" and return the list
of completions as is.  Also, use `try-completions` to simplify the
previous code.