gitweb: hack around CGI's list-context param() handling

gitweb: hack around CGI's list-context param() handling

As of CGI.pm's 4.08 release, the behavior to call
CGI::param() in a list context is deprecated (because it can
be potentially unsafe if called inside a hash constructor).
This causes gitweb to issue a warning for some of our code,
which in turn causes the tests to fail.

Our use is in fact _not_ one of the dangerous cases, as we
are intentionally using a list context. The recommended
route by 4.08 is to use the new CGI::multi_param() call to
make it explicit that we know what we are doing.
However, that function is only available in 4.08, which is
about a month old; we cannot rely on having it.

One option would be to set $CGI::LIST_CONTEXT_WARN globally,
which turns off the warning. However, that would eliminate
the protection these newer releases are trying to provide.
We want to annotate each site as OK using the new function.

So instead, let's check whether CGI provides the
multi_param() function, and if not, provide an
implementation that just wraps param(). That will work on
both old and new versions of CGI. Sadly, we cannot just
check defined(\&CGI::multi_param), because CGI uses the
autoload feature, which claims that all functions are
defined. Instead, we just do a version check.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>