user-manual: fix rendering of history diagrams

user-manual: fix rendering of history diagrams

Asciidoc appears to interpret a backslash at the end of a line as
escaping the end-of-line character, which screws up the display of
history diagrams like

 o--o--o
	\
	 o--...

The obvious fix (replacing "\" by "\\") doesn't work.  The only
workaround I've found is to include all such diagrams in a LiteralBlock.
Asciidoc claims that should be equivalent to a literal paragraph, so I
don't understand why the difference--perhaps it's an asciidoc bug.

Cc: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>