Support defaultKeyCommand for [gpg "ssh"] setting

**Is your feature request related to a problem? Please describe.**

I would like to sign commits with my SSH key.

**Describe the solution you'd like**

Usually I setup my `.gitconfig` as follows:

```
[commit]
gpgsign = true
[gpg]
format = ssh
[gpg "ssh"]
defaultKeyCommand = ssh-add -L
```

This way, it will pick up the ssh key from the ssh-agent as needed.

However it seems that `gitui` does not understand `defaultKeyCommand`. As a consequence, I cannot sign commits on remote computers (which receive the key from the agent).

まず、gitui が [gpg] と [gpg "ssh"] 配下の .gitconfig エントリ、特にコミット署名の設定をどのように読み取るかを追跡します。defaultKeyCommand と ssh-add -L によって署名キーがどのように提供されるべきかを確認し、その後、agent から提供された SSH キーを使ってコミットに署名できることを検証します。

索引モデルが issue の本文から書いたものです。