From: Han-Wen Nienhuys <hanwen@google.com>

REF_LOG_ONLY is used in the transaction preparation: if a symref is involved in
a transaction, the referent of the symref should be updated, and the symref
itself should only be updated in the reflog.

Other ref backends will need to duplicate this logic too, so move it to a
central place.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

Submitted-As: https://lore.kernel.org/git/pull.712.git.1598628333959.gitgitgadget@gmail.com