asciidoctor-extensions.rb: handle "book" doctype in linkgit

asciidoctor-extensions.rb: handle "book" doctype in linkgit

user-manual.txt is the only file we process using the "book" doctype.
When we use AsciiDoc, user-manual.conf ensures that the linkgit macro
expands into something like

  <ulink url="git-foo.html">git-foo(1)</ulink>

in user-manual.xml, which we then process into a clickable link, both in
user-manual.html and user-manual.pdf. With Asciidoctor,
user-manual.conf is ignored (this is expected) and our
Asciidoctor-specific implementation of linkgit kicks in:

  <citerefentry>
    <refentrytitle>git-foo</refentrytitle><manvolnum>1</manvolnum>
  </citerefentry>

This eventually renders as "git-foo(1)", which is not bad, but it
doesn't turn into a link.

Teach our Asciidoctor-specific implementation of the linkgit macro that
if the doctype is "book", we should emit <ulink .../> just like we do
with AsciiDoc. While we're doing this, future-proof by supporting a
"prefix". The implementation in user-manual.conf doesn't support this,
and we don't need this for now because user-manual.txt is the only file
we process this way (and it's immediately in Documentation/).

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>