Hook is extremely slow when pushing large merges

[Balazs Szakmary]

When this plugin is on, the time to push changes is about 100 times more than without the hook. This is very problematic in our main repo. Pushing a merge of 12 days of history (master-> feature branch) takes 5-10 minutes. With the plugin off, the same thing is <5 seconds. I suspect that the plugin iterates over too many commits (pre-existing commits which are part of the push should not be examined) or too many times over the set of commits it looks at.