challenge: Add '--verbose'.

* guix/scripts/challenge.scm (summarize-report): Add #:verbose?
parameter.
[report-hashes]: New procedure.  Use it.
Honor VERBOSE? in the 'match case.
(show-help, %options): Add '--verbose'.
(guix-challenge): Honor it.

doc/guix.texi

@@ -6412,6 +6412,11 @@ The one option that matters is:
 Consider @var{urls} the whitespace-separated list of substitute source
 URLs to compare to.
 
+@item --verbose
+@itemx -v
+Show details about matches (identical contents) in addition to
+information about mismatches.
+
 @end table
 
 @node Invoking guix copy