Force listingblocks to be monospaced in manpages

For the html output we can use a stylesheet to make sure that the
listingblocks are presented in a monospaced font.  For the manpages do
it manually by inserting a ".ft C" before and ".ft" after the block in
question.

In order for these roff commands to get through to the manpage they
have to be element encoded to prevent quoting.
Signed-off-by: NJulian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/asciidoc.conf

@@ -27,7 +27,13 @@ ifdef::backend-docbook[]
 [listingblock]
 <example><title>{title}</title>
 <literallayout>
+ifdef::doctype-manpage[]
+&#10;.ft C&#10;
+endif::doctype-manpage[]
 |
+ifdef::doctype-manpage[]
+&#10;.ft&#10;
+endif::doctype-manpage[]
 </literallayout>
 {title#}</example>
 endif::backend-docbook[]