This improves the documentation for `git_repository_is_empty`
which currently does not accurately describe libgit2's definition
of "empty".

It says that HEAD must point to the "unborn master branch", when
in fact, this is not the case if the repo's `init.defaultBranch`
configuration is set. If it is set, it will check that HEAD points
there. Only if it is not set does it fall back to `master`.