Can't specify a commit hash instead of a branch name

Hey there,

One feature that would be good is the ability to target runs against a specific commit rather than a branch, which would be useful when kicking off automated testing on a busy branch. Currently when you attempt this it will throw out an error that looks something like this:

`Build aborted due to error: Could not fetch specified branch "asdasdasdasdasd" from remote "origin". Command: "git fetch --update-head-ok origin asdasdasdasdasdasd"
Output: ""`

Looking at the [git runner source](https://github.com/box/ClusterRunner/blob/master/app/project_type/git.py#L77) it seems like this may have been in the roadmap at some point, but I couldn't find an issue for it.