fix: support pushurl remote config

[aduh95]

I was trying to upgrade an old project from gh-pages 4.x to 6.x. Let's assume the following git configuration:

$  git remote -v                   
origin  https://github.com/owner/repo.git (fetch)
origin  git@github.com:owner/repo.git (push)

I would expect gh-pages respect that configuration, i.e. pulling using the HTTPS URL (not auth needed for a public repo), and push using the authenticated SSH one. However, gh-pages tries to use the "fetch" one to push, which fails because it's not authenticated.

[tschaub]

Thanks for the proposed change, @aduh95. Can you run npm run lint -- --fix and push the lint fixes?