bug#49603: pcomplete completion for xargs (pcomplete/xargs)

[Karthik Chikmagalur <karthikchikmagalur@gmail.com>]

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Hello,

The function "pcomplete/xargs" (in lisp/pcmpl-unix.el) was set up to
treat xargs with the usage pattern

       xargs [command [initial-arguments]]

Whereas the usage of xargs is actually

       xargs [options] [command [initial-arguments]]

So options to xargs itself were not being completed (assuming
pcomplete-default-completion-function is capable of generating these
completions.)

I added this, treating any string that begins with a "-" following xargs
as an xargs argument and the first string that doesn't as the command
being run by xargs. For example,

       xargs -I{} -0 rm -r
       
-I and -0 will be completed as arguments to xargs (using
 pcomplete-default-completion-function) and rm as a command (using pcomplete-command-completion-function).
 
Commit log entry:

* lisp/pcmpl-unix.el (pcomplete/xargs): Add support for completing xargs
options, including the ability to distinguish them from the command
xargs runs.

Karthik Chikmagalur

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diff -u /home/karthik/.local/share/git/emacs-src/lisp/pcmpl-unix.el /home/karthik/.local/share/git/emacs-src/lisp/pcmpl-unix-new.el
--- emacs-src/lisp/pcmpl-unix.el	2021-07-02 18:50:29.556608640 -0700
+++ emacs-src/lisp/pcmpl-unix-new.el	2021-07-17 00:35:39.417290007 -0700
@@ -82,7 +82,8 @@
 ;;;###autoload
 (defun pcomplete/xargs ()
   "Completion for `xargs'."
-  ;; FIXME: Add completion of xargs-specific arguments.
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (funcall pcomplete-default-completion-function)))
   (funcall pcomplete-command-completion-function)
   (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
 	       pcomplete-default-completion-function)))