* Git: Make signing key id be optional
Thus by default the signing key is selected by commit’s author.
* Git: Add tests for config with and without signing key id
* Git: Format tests for signing key
* Git: Remove default value (null) for signing key
* Git: Update description for signing key
If a user using msmtp to send all their email, it would be preferred if
git used it as well.
The only settings necessary are to set the smtp server to the msmtp
binary and set envelop sender to true, which makes git call msmtp with
the -f flag to set the from address from the email.