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).

Beginne damit nachzuverfolgen, wie gitui die .gitconfig-Einträge unter [gpg] und [gpg "ssh"] liest, insbesondere die Konfiguration zum Signieren von Commits. Prüfe, wie defaultKeyCommand und ssh-add -L den Signaturschlüssel bereitstellen sollten, und verifiziere anschließend, dass Commits mit einem vom Agent bereitgestellten SSH-Schlüssel signiert werden können.

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