PGLog::rewind_divergent_log: log may not contain newhead

Due to split, there may be a hole at newhead.

Fixes: #6722
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
(cherry picked from commit f4648bc6fec89c870e0c47b38b2f13496742b10f)
PG: fix operator<<,log_wierdness log bound warning

Split may cause holes such that head != tail and yet
log.empty().

Fixes: #6722
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
(cherry picked from commit c6826c1e8a301b2306530c6e5d0f4a3160c4e691)