Co-authors not working in submodule

[swalchemist]

% git --version
git version 2.34.1

Reproduction steps:

1. Have a git repo with a git submodule in it.
2. Navigate to the submodule.
3. Set up co-authors, e.g. git duet aa bb.
4. Follow the instructions to set up-coauthors, starting with: set GIT_DUET_CO_AUTHORED_BY to 1 in the environment.
5. Next step from the README: Run git duet. This gives an error:

% git duet
GIT_AUTHOR_NAME='Aaron Albst'
GIT_AUTHOR_EMAIL='aa@example.com'
GIT_COMMITTER_NAME='Bruce Banner'
GIT_COMMITTER_EMAIL='bb@example.com'
open /workspace/parent-repo/src/submodule-repo/.git/hooks/prepare-commit-msg: not a directory
exit status 1

This is failing because .git in a submodule is a regular file, not a directory.

Found by @bruce-ricard and me.