Fix error when attempting to use detached head on existing repo

[chkohner]

Currently if you use a detached head repo (or otherwise have weird branch mappings) the build will fail with a fatal: ref refs/remotes/origin/HEAD is not a symbolic ref error when attempting to get the current branch.

This is a more robust solution that will leave the repo in a detached head if it's not currently associated with a branch (common in a CI environment) and further no longer assumes the remote branch is called origin/${branch} and instead asks git for what the tracked remote branch is.