config/initializers/ascertain_revision.rb

ascertain_revision.rb

# "Ascertain" the revision we're running (so named so it'll come before
# the cache initializer, which uses this info).

def determine_revision
  # Note the revision number we're running, and a
  # more-informative string containing it.
  revision_path = File.dirname(__FILE__) + "/../../REVISION"
  begin
    digits = 8
    if File.exist? revision_path
      mod_date = File.mtime(revision_path)
      number = File.read(revision_path).strip[0...digits]
      extra = mod_date.strftime("%H:%M %a, %b %d %Y").gsub(" 0"," ")
    else
      if File.exist?(".git")
        full_number = `git log -1`.split(" ")[1]
        number = full_number[0...digits]
        extra = `git branch`.split("\n").grep(/^\*/)[0].split(' ')[-1]
      else
        # We don't use --xml anymore, because CentOS's svn doesn't have it.
        number = `svn info`.grep(%r"^Revision: ")[0].split(" ")[1]
        extra = ''
      end
    end
  rescue
    number = '???'
    extra = ''
  end
  details = "#{Rails.env} #{number} #{extra}"
  return full_number, number, details
end

FULL_SOURCE_REVISION_NUMBER, SOURCE_REVISION_NUMBER, SOURCE_REVISION_DETAILS = determine_revision

# Add this to the environment too, so it'll be visible in Hoptoad
ENV["SOURCE_REVISION_DETAILS"] = SOURCE_REVISION_DETAILS

Thanks for sharing.
This helped me put together my solution: http://gist.github.com/405717

you forget to initialize full_number