merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/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" \
@@ -20,9 +23,26 @@ module Git
         end
       end
     end
+
+    class DocumentPostProcessor < Asciidoctor::Extensions::Postprocessor
+      def process document, output
+        if document.basebackend? 'docbook'
+          mansource = document.attributes['mansource']
+          manversion = document.attributes['manversion']
+          manmanual = document.attributes['manmanual']
+          new_tags = "" \
+            "<refmiscinfo class=\"source\">#{mansource}</refmiscinfo>\n" \
+            "<refmiscinfo class=\"version\">#{manversion}</refmiscinfo>\n" \
+            "<refmiscinfo class=\"manual\">#{manmanual}</refmiscinfo>\n"
+          output = output.sub(/<\/refmeta>/, new_tags + "</refmeta>")
+        end
+        output
+      end
+    end
   end
 end
 
 Asciidoctor::Extensions.register do
   inline_macro Git::Documentation::LinkGitProcessor, :linkgit
+  postprocessor Git::Documentation::DocumentPostProcessor
 end