git-describe considers WC dirty incorrectly when using --git-dir

It seems that git-describe always thinks that working copy is dirty if you are not in the WC root and you explicitly specify the .git directory location using --git-dir:

  # set up test repo
  folio:~ chris$ mkdir repo && cd repo
  folio:repo chris$ mkdir text && echo hi > text/hi.txt
  folio:repo chris$ git init . && git add . && git commit -m "text/hi.txt"
  Initialized empty Git repository in /home/chris/repo/.git/
  [master (root-commit) c0edd63] text/hi.txt
  1 file changed, 1 insertion(+)
  create mode 100644 text/hi.txt

  # git-describe from non-root directory
  folio:repo chris$ cd text
  folio:text chris$ git describe --always --dirty
  c0edd63
  folio:text chris$ git --git-dir=../.git describe --always --dirty
  c0edd63-dirty
  folio:text chris$ git --git-dir=$(git rev-parse
--show-toplevel)/.git describe --always --dirty
  c0edd63-dirty

  # git-describe from root directory
  folio:repo chris$ cd ..
  folio:repo chris$ git describe --always --dirty
  c0edd63
  folio:repo chris$ git --git-dir=.git describe --always --dirty
  c0edd63
  folio:repo chris$ git --git-dir=$(git rev-parse
--show-toplevel)/.git describe --always --dirty
  c0edd63