Documentation: asciidoc.conf: always use <literallayout> for [blocktext]

Documentation: asciidoc.conf: always use <literallayout> for [blocktext]

Make the docbook-xsl-no-raw-roff variant match the
no-docbook-xsl-no-raw-roff variant in terms of which XML tag is
used to wrap listing block text (delimited with lines of dashes).

e920b56 (Tweak asciidoc output to work with broken docbook-xsl,
2006-03-05) says docbook-xsl 1.68 needs <literallayout>. This
<screen> usages was in the old, 1.72-only section. But since it
is now the "roff-less" section, it probably makes sense to make it
symmetric with the "roff-ful" section.

Testing done with asciidoc 8.3.1 and docbook-xsl 1.74.0.

Signed-off-by: Chris Johnsen <chris_johnsen@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>