place in your /plugins folder in your jekyll project to use.

internal_link.rb

# Inspiration: Franklin Webber's page_url Jekyll plugin
#
# Usage:
# {% internal_link folder/filename %}
# note: do not include filename extension
#
# Example markdown usage:
# [Link to cool page]({% internal_link sessions/ruby_bootstrap %})
#

module Jekyll
  class Page
    def filename
      filename_without_extension = File.basename(name, File.extname(name)).strip
      if @dir.empty?
        filename_without_extension 
      else
        File.join(@dir, filename_without_extension)
      end
    end

    def internal_path
      File.join(@dir, url)
    end
  end

  class InternalLink < Liquid::Tag
    def initialize(tag_name, filename, tokens)
      super
      @filename = filename.strip
    end

    def render(context)
      context.registers[:site].pages.each do |page|
        if @filename == page.filename || "/#{@filename}" == page.filename
          return page.internal_path
        end
      end
      '#'
    end
  end
end

Liquid::Template.register_tag('internal_link', Jekyll::InternalLink)