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.  Adjust the Makefile to use it by
default.

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

Documentation/Makefile

@@ -179,9 +179,7 @@ ASCIIDOC = asciidoctor
 ASCIIDOC_CONF =
 ASCIIDOC_HTML = xhtml5
 ASCIIDOC_DOCBOOK = docbook45
-ifdef ASCIIDOCTOR_EXTENSIONS_LAB
-ASCIIDOC_EXTRA = -I$(ASCIIDOCTOR_EXTENSIONS_LAB) -rasciidoctor/extensions -rman-inline-macro
-endif
+ASCIIDOC_EXTRA += -I. -rasciidoctor-extensions
 ASCIIDOC_EXTRA += -alitdd='&\#x2d;&\#x2d;'
 DBLATEX_COMMON =
 endif

Documentation/asciidoctor-extensions.rb

@@ -0,0 +1,28 @@
+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'
+          prefix = parent.document.attr('git-relative-html-prefix')
+          %(<a href="#{prefix}#{target}.html">#{target}(#{attrs[1]})</a>\n)
+        elsif parent.document.basebackend? 'docbook'
+          "<citerefentry>\n" \
+            "<refentrytitle>#{target}</refentrytitle>" \
+            "<manvolnum>#{attrs[1]}</manvolnum>\n" \
+          "</citerefentry>\n"
+        end
+      end
+    end
+  end
+end
+
+Asciidoctor::Extensions.register do
+  inline_macro Git::Documentation::LinkGitProcessor, :linkgit
+end