Documentation: render revisions correctly under Asciidoctor

Documentation: render revisions correctly under Asciidoctor

When creating a literal block from an indented block without any sort of
delimiters, Asciidoctor strips off all leading whitespace, resulting in
a misrendered chart.  Use an explicit literal block to indicate to
Asciidoctor that we want to keep the leading whitespace.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>