Add symref validation checks to git refs verify

The git refs verify command runs consistency checks on the reference backends, however, we don't check for validity of symrefs.

See: https://lore.kernel.org/r/aN5mOTbGBcr355E6@pks.im

The command hands over the checks to each reference backend. So, either we handle this check per backend as in this issue for files backend and Add block and reference level consistency check... (#602) • Unassigned for reftable backend, or we combine such common checks.