Support defaultKeyCommand for [gpg "ssh"] setting

[3f6a]

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).