Update Fork so it's clear push refers to the upstream project

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The following discussion from gitlab-ce!24407 should be addressed:

• @godfat started a discussion: (+1 comment)

  I think having attribute push is rather confusing. Where it's pushing to?

  What do you think if we also include a upstream_project from the push?

  def upstream_project
    push.project
  end

  And we should avoid using push directly because it's really confusing.

This refers to qa/qa/resource/fork.rb.

Alternatively, we could rename push to upstream.