Respect git environment variables when committing

**Describe the bug**
According to https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables git uses environment variables as primary source for name and email configuration.

> The final creation of a Git commit object is usually done by git-commit-tree, which uses these environment variables as its primary source of information, falling back to configuration values only if these aren’t present.

**To Reproduce**
Steps to reproduce the behavior:
1. Set:
```
export GIT_COMMITTER_NAME="Your Name"
export GIT_AUTHOR_EMAIL="git@example.com"
export GIT_COMMITTER_EMAIL="git@example.com"
export GIT_AUTHOR_NAME=Your Name"
export EMAIL="git@example.com"
```
2. Commit something with `git commit`
3. See in the logs that the env var values were used
4. Commit something with gitui
5. See in the logs that the git configured user was used instead.

**Expected behavior**
As git docs explains, environment variables should be used as primary source for the git commit command. If the variables are not configured we fall back to git configuration values.

**Screenshots**
Not applicable

**Context (please complete the following information):**
- OS/Distro + Version: Ubuntu 20.04.5
- GitUI Version: gitui 0.22.1
- Rust version: No idea, I use the package from the Nix package manager

**Additional context**
Even though it's suboptimal it's quite common that humans share users on certain machines, when working with Git it's nice to be able to send GIT_* and EMAIL over SSH to Git and gitui 😄
Workaround: Stage with gitui, commit with git (Same workaround as signing commits).

gitui のコミットエントリポイントから開始し、`git commit` に使用する author と committer の identity 値がどのように選択されるかを追跡します。その選択を Git の変数 GIT_AUTHOR_NAME、GIT_AUTHOR_EMAIL、GIT_COMMITTER_NAME、GIT_COMMITTER_EMAIL、EMAIL と比較し、結果を検証するために、報告された環境と設定の相互作用を再現します。

索引モデルが issue の本文から書いたものです。