Disable external diff tools when diffing buffer
Disable external diff tools when diffing buffer

There is a bug that causes `:git show-diff` to fail when using an external diff, for example difftastic.

This change ensures that we don't use an external diff tool when diffing the current buffer.
casimir/kakoune,mawww/kakoune,mawww/kakoune,casimir/kakoune,jkonecny12/kakoune,casimir/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,casimir/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune