Create a new action signature with default user and now timestamp.

This looks up the user.name and user.email from the configuration and uses the current time as the timestamp, and creates a new signature based on that information. It will return GIT_ENOTFOUND if either the user.name or user.email are not set.

Note that these do not examine environment variables, only the configuration files. Use git_signature_default_from_env to consider the environment variables.