Inconsistent behavior with git submodules

[protometa]

When using difftastic (I'm on version 0.61.0 on NixOs) with git submodules, git diff --submodule will only show the short diff of submodules (old/new hash). The usual --submodule=log format can be shown only by disabling difftastic external diff with git diff --submodule --no-ext-diff. The diff format can be shown with git diff --submodule=diff --no-ext-diff, in which case the submodule diff will actually be shown with diftatstic while the parent repo diff will be shown with standard git diff.