git is slow when the index file is large and has to be rewritten each time
a file is changed. To speed this up, added a journal where changes are
recorded before being fed into the index file and committed to the
git-annex branch. The entire journal can be fed into git with just 2
commands, and only one write of the index file.
to avoid some issues with git on OSX with the mixed-case directories. No
migration is needed; the old mixed case hash directories are still read;
new information is written to the new directories.
