add a "quick fail" to appveyor in case a new push to a PR was made #6004

This helps in cases where someone pushes new commits to a PR and the old commits were not yet built. As appveyor takes quite a lot time (only one build at a time), this can reduce the time until the last commit in a PR is actually built and also reduce the time until other (unrelated) PRs are built.

Actual code here: https://github.com/ogrisel/python-appveyor-demo/pull/37/files