PGLog::proc_replica_log: _merge_divergent_entries based on truncated olog

PGLog::proc_replica_log: _merge_divergent_entries based on truncated olog

We can't merge using the primary's log since we haven't decided whether
to send them a complete log yet.  Thus, merge based on the truncated olog
rather than the primary's log.  This is a consequence of the division
between trimming divergent entries in peering/unfound search and sending
a complete log to actual members of the actingbackfill set in activate().
_merge_divergent_entries on the truncated log and add_next_event() on the
newer entries result in the same missing/log regardless of the order in
which they are performed.

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