Documentation: fix build with Asciidoctor 2

Our documentation toolchain has traditionally been built around DocBook
4.5.  This version of DocBook is the last DTD-based version of DocBook.
In 2009, DocBook 5 was introduced using namespaces and its syntax is
expressed in RELAX NG, which is more expressive and allows a wider
variety of syntax forms.

Asciidoctor, one of the alternatives for building our documentation,
moved support for DocBook 4.5 out of core in its recent 2.0 release and
now only supports DocBook 5 in the main release.  The DocBoook 4.5
converter is still available as a separate component, but this is not
available in most distro packages.  This would not be a problem but for
the fact that we use xmlto, which is still stuck in the DocBook 4.5 era.

xmlto performs DTD validation as part of the build process.  This is not
problematic for DocBook 4.5, which has a valid DTD, but it clearly
cannot work for DocBook 5, since no DTD can adequately express its full
syntax.  In addition, even if xmlto did support RELAX NG validation,
that wouldn't be sufficient because it uses the libxml2-based xmllint to
do so, which has known problems with validating interleaves in RELAX NG.

Fortunately, there's an easy way forward: ask Asciidoctor to use its
DocBook 5 backend and tell xmlto to skip validation.  Asciidoctor has
supported DocBook 5 since v0.1.4 in 2013 and xmlto has supported
skipping validation for probably longer than that.

We also need to teach xmlto how to use the namespaced DocBook XSLT
stylesheets instead of the non-namespaced ones it usually uses.
Normally these stylesheets are interchangeable, but the non-namespaced
ones have a bug that causes them not to strip whitespace automatically
from certain elements when namespaces are in use.  This results in
additional whitespace at the beginning of list elements, which is
jarring and unsightly.

We can do this by passing a custom stylesheet with the -x option that
simply imports the namespaced stylesheets via a URL.  Any system with
support for XML catalogs will automatically look this URL up and
reference a local copy instead without us having to know where this
local copy is located.  We know that anyone using xmlto will already
have catalogs set up properly since the DocBook 4.5 DTD used during
validation is also looked up via catalogs.  All major Linux
distributions distribute the necessary stylesheets and have built-in
catalog support, and Homebrew does as well, albeit with a requirement to
set an environment variable to enable catalog support.

On the off chance that someone lacks support for catalogs, it is
possible for xmlto (via xmllint) to download the stylesheets from the
URLs in question, although this will likely perform poorly enough to
attract attention.  People still have the option of using the prebuilt
documentation that we ship, so happily this should not be an impediment.

Finally, we need to filter out some messages from other stylesheets that
occur when invoking dblatex in the CI job.  This tool strips namespaces
much like the unnamespaced DocBook stylesheets and prints similar
messages.  If we permit these messages to be printed to standard error,
our documentation CI job will fail because we check standard error for
unexpected output.  Due to dblatex's reliance on Python 2, we may need
to revisit its use in the future, in which case this problem may go
away, but this can be delayed until a future patch.

The final message we filter is due to libxslt on modern Debian and
Ubuntu.  The patch which they use to implement reproducible ID
generation also prints messages about the ID generation.  While this
doesn't affect our current CI images since they use Ubuntu 16.04 which
lacks this patch, if we upgrade to Ubuntu 18.04 or a modern Debian,
these messages will appear and, like the above messages, cause a CI
failure.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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