Allow filter `since` a commit

[harley]

Thanks for the cool utility 👍 This would save a lot of time whe you run rewrite the second time because we only need to parse new commits. Git filter branch accepts range notation http://stackoverflow.com/a/15250521

[davidfokkema]

Nice to hear you find it useful! I'm very busy with other things; if you can find the time to create a PR, I'll certainly look at it. Otherwise, this goes on the stack, I'm afraid.