For easy git usage I created these two scripts in my PATH instead of using using git config:

git-difft:

  #!/bin/sh
  GIT_EXTERNAL_DIFF=difft git diff "$@"

git-showt:

  #!/bin/sh
  GIT_EXTERNAL_DIFF=difft git show --ext-diff "$@"

Then you can run "git difft …" or "git showt …" if you want to use it.