Don't error when profiling an unborn branch

[danielsdeleo]

As described in #364 when you create a repo (git init) but don't have a commit, you're on an unborn master branch. Commands such as git rev-parse HEAD will fail in such a scenario, which causes most Policyfile commands to fail (when attempting to extract git information about the cookbook). There does not seem to be a canonical way to detect that you're on an unborn branch, git uses the same exit code for all failures, and the error messages printed to stderr do not seem to be stable across git versions. So I settled on feeding the output of git symbolic-ref -q HEAD to show-ref --verify $SYMBOLIC_REF (we expect the first command to succeed and the second command to fail on an unborn branch).

fixes #364

@chef/client-core @tyler-ball

[tyler-ball]

👍 LGTM - and it passed on appveyor, woohoo!