After consultation with Jim, we agreed that Full's behavior vis object
serial numbers after a version commit/abort are correct.  The new
revision should get the serial number of the transaction that did the
commit/abort.

FileStorage is actually broken here; the new revision gets the same
serial number as the last in-version revision.  However, the way the
tests are written, it works for either storage, so I'm just updating
some comments to explain the situation.

Note: Jim also explained that history()'s "version" argument should be
ignored.  Doesn't make much difference to these tests though.