user-manual.txt: render ASCII art correctly under Asciidoctor
This commit is similar to
379805051d ("Documentation: render revisions
correctly under Asciidoctor", 2018-05-06) and is a no-op with AsciiDoc.
When creating a literal block from an indented block without any sort of
delimiters, Asciidoctor strips off all leading whitespace, resulting in
a misrendered ASCII drawing. Use an explicit literal block to indicate
to Asciidoctor that we want to keep the leading whitespace. Drop the
common indentation for all lines to make this a no-op with AsciiDoc.
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>