docs: default to more modern toolset

docs: default to more modern toolset

When the ASCIIDOC8 and ASCIIDOC_NO_ROFF knobs were built,
many people were still on asciidoc 7 and using older
versions of docbook-xsl. These days, even the almost
2-year-old Debian stable needs these knobs turned.

So let's turn them by default. The new knobs ASCIIDOC7 and
ASCIIDOC_ROFF can be used to get the old behavior if people
are on older systems.

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