D3649: serve: add option print-url

[Yuya Nishihara]

>      ('t', 'templates', '', _('web templates to use'), _('TEMPLATE')),
>      ('', 'style', '', _('template style to use'), _('STYLE')),
>      ('6', 'ipv6', None, _('use IPv6 in addition to IPv4')),
> -    ('', 'certificate', '', _('SSL certificate file'), _('FILE'))]
> +    ('', 'certificate', '', _('SSL certificate file'), _('FILE')),
> +    ('', 'print-url', None, _('start and print only the URL'))]
>       + subrepoopts,
>      _('[OPTION]...'),
>      optionalrepo=True)
> @@ -4798,6 +4799,10 @@
>      opts = pycompat.byteskwargs(opts)
>      if opts["stdio"] and opts["cmdserver"]:
>          raise error.Abort(_("cannot use --stdio with --cmdserver"))
> +    if opts["print_url"] and ui.verbose:
> +        raise error.Abort(_("cannot use --print-url with --verbose"))
> +    if opts["print_url"]:
> +        opts['daemon'] = True

It's surprising that `hg serve --print-url` spawns a daemon process!