challenge: Add "--diff".

challenge: Add "--diff".

* guix/scripts/challenge.scm (dump-port*): New variable.
(archive-contents, store-item-contents, narinfo-contents)
(differing-files, report-differing-files): New procedures.
(summarize-report): Add #:report-differences and call it.
(show-help, %options): Add "--diff".
(%default-options): Add 'difference-report' key.
(report-differing-files): Parameterize CURRENT-TERMINAL-COLUMNS and pass
 #:report-differences to 'summarize-report'.
* guix/tests/http.scm (%local-url): Add optional argument.
(call-with-http-server): Fix docstring typo.
* tests/challenge.scm (query-path-size, make-narinfo): New procedures.
("differing-files"): New test.
* doc/guix.texi (Invoking guix challenge): Document "--diff".