pull: Use (guix inferior) to display new and upgraded packages.

* guix/scripts/pull.scm (display-profile-content): Call
'display-generation'.
(display-new/upgraded-packages, display-profile-content-diff): New
procedures.
(process-query)[list-generation]: Remove.
[list-generations]: New procedure.
Adjust accordingly.
* doc/guix.texi (Invoking guix pull): Update example of '-l'.