Fix relative links in repo docs

Relative links always need to point to one level below the current path,
as nanoc path end on a trailing slash.

lib/filters/normalize_links.rb

@@ -17,9 +17,10 @@ class NormalizeLinks < ::Nanoc::Filter
         when link['href'].start_with?(DOMAIN)
           link['href'][DOMAIN.size..-1]
         when link['href'].start_with?('/')
-          link_to(link['href'], config)
+          # TODO(ts): It's not guaranteed that a repository is hosted on Github.
+          github_link_to(link['href'], config)
         when link['href'].include?('.md')
-          link['href'].gsub(/\.md($|#)/, '/\\1')
+          relative_link_to(link['href'])
         else
           link['href']
         end
@@ -28,12 +29,16 @@ class NormalizeLinks < ::Nanoc::Filter
     doc.to_s
   end
 
-  # TODO(ts): It's not guaranteed that a repository is hosted on Github.
-  def link_to(file, config)
+  def github_link_to(file, config)
     base = config[:repository]
     if base.end_with?('.git')
       base = base[0..-5]
     end
     File.join(base, 'blob', config[:refspec], file)
   end
+
+  def relative_link_to(link)
+    # All nanoc pages end on a trailing slash.
+    File.join("../", link.gsub(/\.md($|#)/, '/\\1'))
+  end
 end