PG,PGLog: replace _merge_old_entry with _merge_object_divergent_entries

PG,PGLog: replace _merge_old_entry with _merge_object_divergent_entries

The _merge_old_entry structure had trouble distinguishing between the
following cases:

missing: foo, 1,1
merge_old_entry modify 1,1 0,0
merge_old_entry modify 1,2 1,1

and
merge_old_entry modify 1,2 1,1

In the first case, we should end up with foo removed from missing
at the end.  In the second, we need foo added to missing at 1,1.
It's far simpler to present all of the divergent entries for a single
object at once.

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