COMMIT_EDITMSG is not updated

Hello guys and thank you for a wonderful package!

I am experiencing some issues though if commiting from within a submodule directory.
```
(node:16959) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: ENOTDIR: not a directory, open '/home/ubuntu/path/to/repository/submodule/.git/COMMIT_EDITMSG' [edited for bug report]
(node:16959) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
```

Nonetheless script is definitely right because hooks for submodules are located under `../.git/modules/submodule/hooks` and therefore `COMMIT_EDITMSG` file would never appear under `submodule/.git/COMMIT_EDITMSG` and `submodule/.git` is file, that's true as well ;)

I cannot give a time to resolve it by myself at the moment but will hopefully provide a solution somehow later as a contributor.

Commencez par reproduire un commit depuis un sous-module Git et suivez le traitement de COMMIT_EDITMSG. Comparez le chemin défaillant submodule/.git/COMMIT_EDITMSG avec l’emplacement des hooks sous ../.git/modules/submodule/hooks ; le travail est terminé lorsque le flux du commit n’essaie plus d’ouvrir le chemin invalide et gère l’opération sans rejet non géré.

Rédigé par le modèle d'indexation à partir du texte de l'issue.