Support ssh-agent and external sign binaries for commit signing with ssh

[chirpcel]

This Pull Request fixes/closes #2188.

It changes the following:

• Respect configuration for alternative signing binary (for example 1Password ssh agent)
• Using ssh-keygen for ssh signing operation to support ssh-agent
  • additional: sharing same implementation for default & configured binary

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[chirpcel]

There's one drawback. Like before, we don't support encrypted private keys which are not loaded to agent. The error message isn't as clear as before, but from a functionality point of view it isn't different. I would make a new issue for this behavior and take a look into supporting encrypted private keys.

[extrawurst]

The error message isn't as clear as before

cant we make it as clear as before and then followup

supporting encrypted private keys.

[chirpcel]

Actually I didn't figured out how to capture the passphrase prompt, as it's spawned directly on the tty.
But I just have an Idea, maybe we could provide an empty passphrase as arg to skip the password prompt and map the "wrong passphrase" error to the previous error message.
I'll take a look at it today.

[chirpcel]

@extrawurst Worked, we now have a meaningful error message. I tested with 1Password SSH-Agent, Default SSH-Agent, non encrypted keys on disk and encrypted keys on disk.