unit tests for PGLog::rewind_divergent_log

The tests covers 100% of the LOC of rewind_divergent_log. There are
three situations :

 * throw an assert because the data is inconsistent

 * special case when the entire logs is divergent

 * regular workflow where all divergent entries are run to
   merge_old_entry

http://tracker.ceph.com/issues/5213 refs #5213

Signed-off-by: Loic Dachary <loic@dachary.org>