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.

首先，在 Git 子模組中重現一次 commit，並追蹤 COMMIT_EDITMSG 的處理方式。比較失敗的路徑 submodule/.git/COMMIT_EDITMSG 與 ../.git/modules/submodule/hooks 下的 hooks 位置；當 commit 流程不再嘗試開啟無效路徑，並且能在沒有未處理 rejection 的情況下處理該操作時，即表示完成。

由索引模型根據 Issue 內容生成。