Repository default falls back to github user from git config.

Previously, `repository` key default guessed `process.env.USER{NAME}` as the user's GitHub handle, without checking the user's git config for a `github.user` key, even though that was checked later in the `sanitize` function, when populating the `git_user` key. This commit does the check earlier for a smarter default in case no git remote URL is found from `git.origin`.