Documentation: implement linkgit macro for Asciidoctor

AsciiDoc uses a configuration file to implement macros like linkgit,
while Asciidoctor uses Ruby extensions.  Implement a Ruby extension that
implements the linkgit macro for Asciidoctor in the same way that
asciidoc.conf does for AsciiDoc.

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

Documentation/extensions.rb

@@ -0,0 +1,39 @@
+require 'asciidoctor'
+require 'asciidoctor/extensions'
+
+module Git
+  module Documentation
+    class LinkGitProcessor < Asciidoctor::Extensions::InlineMacroProcessor
+      use_dsl
+
+      named :chrome
+
+      def process(parent, target, attrs)
+        if parent.document.basebackend? 'html'
+          generate_html(parent, target, attrs)
+        elsif parent.document.basebackend? 'docbook'
+          generate_docbook(parent, target, attrs)
+        end
+      end
+
+      private
+
+      def generate_html(parent, target, attrs)
+        section = attrs.has_key?(1) ? "(#{attrs[1]})" : ''
+        prefix = parent.document.attr('git-relative-html-prefix') || ''
+        %(<a href="#{prefix}#{target}.html">#{target}#{section}</a>\n)
+      end
+
+      def generate_docbook(parent, target, attrs)
+        %(<citerefentry>
+<refentrytitle>#{target}</refentrytitle><manvolnum>#{attrs[1]}</manvolnum>
+</citerefentry>
+)
+      end
+    end
+  end
+end
+
+Asciidoctor::Extensions.register do
+  inline_macro Git::Documentation::LinkGitProcessor, :linkgit
+end