A root commit is a commit whose branch (usually what HEAD points to)
doesn't exist (yet). This situation can happen when the commit is the
first after 1) a repository is initialized or 2) a orphan checkout has
been performed.

Take this opportunity to remove the symbolic link check, as
git_reference_resolve works on OID refs as well.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>