Fix Pcompletion of "tar" when using unrecognized arguments

Fix Pcompletion of "tar" when using unrecognized arguments

Previously, arguments to tar like "--warning=no-timestamp" would cause
Pcompletion to hang (bug#58921).

This simplifies the logic flow by moving all the cases for "--"
arguments inside the THEN form of '(if (pcomplete-match "^--" 0)', and
for all "-" arguments inside the ELSE form.

* lisp/pcmpl-gnu.el (pcmpl-gnu--tar-long-options): New variable.
(pcomplete/tar): Properly handle completion of arguments that look
like "--ARG=", even if they're not recognized by this function.