Currently running `git-webkit pr` calls `check-webkit-style` by default. However, both commands run `Branch.branch_point` for the commit being pushed, and `Branch.branch_point` is famously slow (e.g., bug 256855). Instead of spending ~15s doing this twice, we should spend ~7.5s doing it once, and `git-webkit pr` should simply tell `check-webkit-style` what commit to use as the branch point.
<rdar://problem/109558732>