What does "new_rev" mean in _sync metadata?

(Moved from https://github.com/couchbase/sync_gateway/issues/1624)

User hitales asked:

I found a document in couchbase with “new_rev”, and this document can not synchronize to couchbase lite forever, i don’t know why; one the lite side, the document is not in conflict list.

So, What does “new_rev” means in _sync? how can i resolve such document?

For example:
“new_rev”: “7-89c099875bdeb20239dc68c88ab07727”,
“rev”: “11-4fac5074eb7189ca1a836ae84bd8edf7”

The new_rev property is set when a new revision is added that isn’t the active revision for a document.

In this case, it looks like the document has at least two conflicting branches, and revision 7-89c099875bdeb20239dc68c88ab07727 was added to one of those branches. However, the already existing rev 11-4fac5074eb7189ca1a836ae84bd8edf7 is still the active revision.