Documentation: inline user-manual.conf

Documentation: inline user-manual.conf

When generating our user manual we set up a bit of extra configuration
compared to our normal configuration. This is done by having an extra
"user-manual.conf" file that Asciidoc seems to pull in automatically due
to matching filenames with "user-manual.txt". This dependency is quite
hidden though and thus easy to miss. Furthermore, it seems that Asciidoc
does not know to pull it in for out-of-tree builds where we use relative
paths.

The setup in AsciiDoctor is somewhat different: instead of having two
sets of configuration, we condition the use of manual-specific configs
based on whether the document type is "book". And as we only build our
user manual with that type this is sufficient.

Use the same trick for our user manual by inlining the configuration
into "asciidoc.conf.in" and making it conditional on whether or not
"doctype-book" is defined.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>