xorm pairs declared and existing indexes by column set, ignoring name
and column order, so on every sync the `action` table could drop one of
`c_u` and `c_u_d`, and never recreate a missing one. Tables with such
index pairs are now synced with IgnoreIndices and their plain indexes are
reconciled by name.
Fix https://github.com/go-gitea/gitea/issues/39244