git difftool ignores textconv driver -

git diff take account textconv driver, git difftool -d --textconv not. why? how fix it?

edit: more informations

my ~/.gitconfig contains among other settings:

[diff]     tool = default-difftool [difftool "default-difftool"]     cmd = vim -f '+next' '+execute \"dirdiff\" argv(0) argv(1)' $local $remote '+syntax off' [diff "ipynb"]     textconv = nbcatsrc 

and .gitattributes is:

*.ipynb diff=ipynb