merge-ort: extract handling of priv member into reusable function

merge-ort: extract handling of priv member into reusable function

In preparation for a subsequent commit which will ensure we do not
forget to maintain our invariants for the priv member in error
codepaths, extract the necessary functionality out into a separate
function.  This change is cosmetic at this point, and introduces no
changes beyond an extra assertion sanity check.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>