debugdeltachain: detect a special case where parents are "skipped" See inline comment for details, this is a case where the delta is neither against p1 or p2, Yet it is still a simple delta part of a simple chain. We now display them as `skip1/skip2` instead of `other`.

debugdeltachain: document the possible value for deltatype So that one can understand what is displayed by the command.

debugdeltachain: also display p1/p2 Looking at the parents is a common need when trying to understanding why a delta was chosen, having it readily available helps a lot.

debugdeltachain: glob variance of "test-generaldelta" We mostly care about generaldelta happening, the exact details of storage size variation is not really important so we can glob it instead of having multiple lines for each variances. This will make updating the output of the command simpler.