checkpatch: Exempt long URLs

Sometimes, we want to refer to really long URLs, but checkpatch
balks, and we have to manually bypass the check.  URL shorteners
may be nice at reducing long links, but it's hard to guarantee the
shortened link will live as long as the real target, and it is
also nice to see the original target without having to load the
shortened URL through a browser.  So exempt a line containing
only a URL from the long-line syntax check.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180222215838.18223-1-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>