Give the marker of Fbe.repeatedly an _id when it has none

[Thayorns]

Fbe.repeatedly created its marker with fb.insert and saved it with Fbe.overwrite. On a plain Factbase the marker had no _id, which Fbe.overwrite needs to replace a property that is already there, so the first run worked and the second one failed with "There is no _id in the fact, cannot use Fbe.overwrite". The existing tests all wrap the factbase in Fbe.fb, which assigns ids, so none of them saw it.

The marker now gets a random _id when it has none, the same way Fbe.iterate and Fbe::Tombstone handle their own facts. A marker that already has an _id, which is always the case under Fbe.fb, keeps it.

The tests run the judge twice on a plain Factbase, update an old marker that was stored without an _id, and check that a marker with an _id keeps it. With one more line in it, Fbe.repeatedly goes over the Metrics/AbcSize limit, which it was already close to (49.7 of 50), so it now carries # rubocop:disable Metrics/AbcSize like Fbe.fb and Fbe.overwrite.

Closes #1247

[Thayorns]

@yegor256 take a look please, happy to clarify anything about the change.