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.

Start by reproducing a commit from within a Git submodule and trace the handling of COMMIT_EDITMSG. Compare the failing submodule/.git/COMMIT_EDITMSG path with the hooks location under ../.git/modules/submodule/hooks; done means the commit flow no longer tries to open the invalid path and handles the operation without an unhandled rejection.

Written by the indexing model from the issue text.