Rotate only if there will be a new <interval>.0 folder

[WKarel]

A copy of #92, which seemed to be merged since 2015, but has never been. Re-opened as a new PR following this advice.

With this bugfix, gaps like a missing .0 folder will be prevented.
See bug #18.

replace #34

[WKarel]

@myrdd , any comments?

[sgpinkus]

Actually not sure about this still.

1: As @nkadel commented on #34

In some environments, you want to discard all backups past a certain age, even at the cost of braking hardlinking. But in general, I think it's a really good idea.

2: should it be an error or just "nothing to do" / null-op.

3: It really should have an option like strict_rotation_checks 0|1 or even strict_rotation_checks 0|1|'null-op' where null-op means do checks but don't error if can't rotate. We could do a version bump but an option is better.

4: It still doesn't solve destructive rotation bug #3 and link_dest is 0 by default. Adding this as default behavior could make that bug/gotcha even harder to understand.

Summary: having an option strict_rotation_checks 0|1|'null-op' (or just strict_rotation_checks 0|1|2 with doc ..) kills concerns 1,2,3, so I think we should go for that before merging this. 4 can be handled separately if fix can't be easily added with this PR.