PGLog::merge_old_entry: fix prior_version > info.log_tail case

If the object is not missing, we must have rolled back the prior
log entry.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: rollback if possible in the oe.version > ne.version case

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: split out _merge_log as a static method

Signed-off-by: Samuel Just <sam.just@inktank.com>
working
working