elisp: Support new options for "guix pack"

Which were added by:
<http://git.savannah.gnu.org/cgit/guix.git/commit/?id=fd214f1522063905021a297dab1ac4d85d94ad83> and
<http://git.savannah.gnu.org/cgit/guix.git/commit/?id=a0f352b30f4869a7af7017b8a5011ac7602dd115>.

* elisp/guix-command.el (guix-command-improve-pack-argument): Use "P"
  for "--entry-point" ("e" is for "--expression").
  Complete file name for "--root".
* elisp/guix-pcomplete.el (guix-pcomplete-complete-option-arg): Likewise.

elisp/guix-command.el

@@ -290,8 +290,10 @@ to be modified."
 (guix-command-define-argument-improver
     guix-command-improve-pack-argument
   '(("--compression" :fun guix-read-compressor-name)
+    ("--entry-point" :char ?P)
     ("--format" :fun guix-read-pack-format-name)
     ("--localstatedir" :char ?L)
+    ("--root" :fun guix-read-file-name)
     ("--save-provenance" :char ?p)
     ;; "--symlink" is not completed as it should be "FILE-NAME=TARGET".
     ;; ("--symlink" :fun guix-read-file-name)

elisp/guix-pcomplete.el

@@ -278,10 +278,11 @@ INPUT is the current partially completed string."
         (complete* (guix-compressor-names)))
        ((option? "-f" "--format")
         (complete* (guix-pack-format-names)))
-       ;; Although the argument should be "FILE-NAME=TARGET", it is
-       ;; still better to complete the FILE-NAME than to complete
-       ;; nothing.
-       ((option? "-S" "--symlink")
+       ;; Although the argument for "--symlink" should be
+       ;; "FILE-NAME=TARGET", it is still better to complete FILE-NAME
+       ;; than to complete nothing.
+       ((or (option? "-r" "--root")
+            (option? "-S" "--symlink"))
         (complete* (pcomplete-entries)))))
 
      ((command? "potluck")