No such thing as an orphan branch

Unfortunately git-core uses the term "unborn branch" and "orphan
branch" interchangeably. However, "orphan" is only really there for
the checkout command, which has the `--orphan` option so it doesn't
actually create the branch.

Branches never have parents, so the distinction of a branch with no
parents is odd to begin with. Crucially, the error messages deal with
unborn branches, so let's use that.
Merge pull request #1856 from libgit2/cmn/no-orphans

No such thing as an orphan branch