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>

Documentation/asciidoctor-extensions.rb

@@ -9,8 +9,11 @@ module Git
       named :chrome
 
       def process(parent, target, attrs)
-        if parent.document.basebackend? 'html'
-          prefix = parent.document.attr('git-relative-html-prefix')
+        prefix = parent.document.attr('git-relative-html-prefix')
+        if parent.document.doctype == 'book'
+          "<ulink url=\"#{prefix}#{target}.html\">" \
+          "#{target}(#{attrs[1]})</ulink>"
+        elsif parent.document.basebackend? 'html'
           %(<a href="#{prefix}#{target}.html">#{target}(#{attrs[1]})</a>)
         elsif parent.document.basebackend? 'docbook'
           "<citerefentry>\n" \