The `tag.gpgSign` config option was added in Git 2.23.0 and seems like
it should be set in addition to `commit.gpgSign` when
`programs.git.signing.signByDefault` is enabled
* 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