Tweak asciidoc output to work with broken docbook-xsl

Tweak asciidoc output to work with broken docbook-xsl

docbook-xsl v1.68 incorrectly converts "<screen>" from docbook to
manpage by not rendering it verbatim. v1.69 handles it correctly, but
not many current popular distributions ship with it.

asciidoc by default converts "listingblock" to "<screen>". This change
causes asciidoc in git to convert "listingblock" to "<literallayout>", which
both old and new docbook-xsl handle correctly.

The difference can be seen in any manpage which includes a multi-line
example, such as git-branch.

[jc: the original patch was an disaster for html backends, so I made
 it apply only to docbook backends. ]

Signed-off-by: Junio C Hamano <junkio@cox.net>