This was one of those days where I somehow end up dealing with tricky
filename encoding problems all day.

First, worked around inability for concurrent-output to display unicode
characters when in a non-unicode locale. The normal trick that git-annex
uses doesn't work in this case. Since it only affected -J, I decided to
make git-annex detect the problem and make -J behave as if it was not built
with the concurrent-output feature. So, it just doesn't display concurrent
output, which is better than crashing with an encoding error.

The other problem affects v6 repos only. Seems that not all Strings will
round trip through a persistent sqlite database. In particular, unicode
surrogate characters are replaced with garbage. This is really [a bug in
persistent](https://github.com/yesodweb/persistent/issues/540).
But, for git-annex's purposes, it was possible to work around it,
by detecting such Strings and serializing them differently.

Then I had to enhance `git annex fsck` to fix up repositories that were
affected by that problem.