TestPGLog::proc_replica_log: call doesn't adjust olog

We don't adjust head with proc_replica_log, don't assert that
it's been adjusted.

Signed-off-by: Samuel Just <sam.just@inktank.com>
TestPGLog: check on last_update in run_test_case

Signed-off-by: Samuel Just <sam.just@inktank.com>
TestPGLog: add a test case verifying case where we have the prior_version

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog::_merge_object_divergent_entries: handle missing.have == prior_version

Fixes: #7658
Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog::proc_replica_log: select divergent log entries correctly

Looking for an entry in olog which matches one of ours might add
extra divergent entries.  Instead, do what merge_log does and
walk back through the auth log looking for an entry in olog.

Fixes: 7657
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge branch 'wip-div' into wip-sam-testing