Mark user-manual as UTF-8

Mark user-manual as UTF-8

There have been several complaints against k.org's user-manual
page.  The document is generated in ISO-8859-1 by the xsltproc
toolchain (I suspect this is because released docbook.xsl we use
has xsl:output element that says the output is ISO-8859-1) but
server delivers it with "charset=UTF-8", and all h*ll breaks
loose.

This attempts to force UTF-8 on the generating end.

Signed-off-by: Junio C Hamano <gitster@pobox.com>