treat dead repos as 0 size

treat dead repos as 0 size

With this, git annex log --totalsizes can be compared with
git-annex info's "combined annex size of all repositories"
to double-check it works correctly.

In my sound repo, the two match.

In my big repo, the two report slightly different sizes,
with the former being 1.3 gb smaller than the latter.
I don't know the reason for this disreprency. Given the 30+tb size of
the repo, it's a small difference.

It seems possible that a bug in an old version of git-annex could
explain it. Eg, if an old git-annex lost a line when updating trust.log
or a location log in a merge, git-annex info would see only what it
replaced it with, while git-annex log will see the previous value as
well.

Sponsored-by: Leon Schuermann on Patreon