Failovers and switchover are really annoying, because:

1. FULL backup must be taken
2. user is forced to handle this in their scripts.

So it should be allowed to take incremental backup even if timeline ID is advanced since parent backup was taken.

TODO:

1. tests
2. documentation

UPDATE:
This is impossible without WAL archive, because we need information about timeline switchpoint to correctly choose parent, located on previous timeline:

            /--------------X(we are here)
t1 -----A---*-----B---->

Choosing parent simply by time sorting will produce incorrect incremental backup.