remove print
forget --drop-dead: Completely removes mentions of repositories that have been marked as dead from the git-annex branch.

Wrote nice pure transition calculator, and ugly code to stage its results
into the git-annex branch. Also had to split up several Log modules
that Annex.Branch needed to use, but that themselves used Annex.Branch.

The transition calculator is limited to looking at and changing one file at
a time. While this made the implementation relatively easy, it precludes
transitions that do stuff like deleting old url log files for keys that are
being removed because they are no longer present anywhere.