user-manual: ensure generated manual references stylesheet

user-manual: ensure generated manual references stylesheet

The generated user manual is rather hard to read thanks to the lack of
the css that's supposed to be included from docbook-xsl.css.

I'm totally ignorant of the toolchain; grubbing through xmlto and
related scripts, the easiest way I could find to ensure that the
generated html links to the stylesheet is by calling xsltproc directly.
Maybe there's some better way.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>