It should continue to skip following operations.
The test from rev14 to rev0 fails if it doesn't call continue and append
all revisions of the same main rev to the list.
It should skip last empty generation when the key is just tombstoned.
The rev15 and rev16 in the test fails if it doesn't skip last empty generation
and find previous generations.
Before this commit, it will return wrong create index, mod index.
It lets findGeneration return error when rev is at the gap of two
generations. This leads to the change of compact() code.
