We're having to do pretty fragile stuff parsing the output of the git-branches porcelain command; we probably should be using `git-for-each-ref` instead. That said, if we're too old of a version of git (<2.13) , then we don't have --contains there (nor --format on git-branch).
<rdar://problem/94004623>
Pull request: https://github.com/WebKit/WebKit/pull/3150