A dead key is both not present at the location that thinks it has a copy,
and also is assumed to probably not be present anywhere else. Although
there may be lurking disconnected repos that somehow still have a copy.
Suprisingly few changes needed for this! This is because the presence log
code only really concerns itself with keys that are present, and dead keys
are not present.
Note that both the location and web log can be parsed as having a dead key.
I don't see any value to having keys listed as dead in the web log, but
since it doesn't change any behavior, there was no point in not parsing it.
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.
