Allow to test PR on synchronize

When new commit is pushed to a target branch, github sends us
synchronize event. We haven't tested this commit, unless head was
changed, but it is a valid use case - when master changes, PR may no
longer pass, so it's good to retest it.

This commit allows to enable such a behavior for a specific repository
slug.

Piotr Sarnacki authored and committed

Nothing here #2684.3 failed

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