docs: manpage XML depends on asciidoc.conf

docs: manpage XML depends on asciidoc.conf

When building manual pages, the source text is transformed to XML with
AsciiDoc before the man pages are generated from the XML with xmlto.

Fix the dependencies in the Makefile so that the XML files are rebuilt
when asciidoc.conf changes and not just the manual pages from
unchanged XML, and move the dependencies from a recipeless rule to the
rules with commands that use asciidoc.conf to make the dependencies
easier to understand and maintain.

Reported-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Tested-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>