merge: under new algorithm, if an upload tag does not have a changelog parent, merge start breaks

[Nish Aravamudan]

This is the case for src:ntp currently.

The new importer will drop these 'incorrect' upload tags on reimports. But if we change the algorithm again in the future to not drop them, then we need to fix merge to properly detect the situation where it can't 'start' automatically.

Specifically, I think we can walk old/ubuntu backwards and if we find an upload tag (which will be the same as an import tag in the new algorithm), we walk the upload tag backwards until we get to a point in time where we have hit another import tag (and check it is the one we expect?) or debian/changelog in the walked commit goes "before" the expected import tag. In the latter case, we assume they've given us a rich history with a different ancestry than the importer intentionally and merge start can't be used.