Made `git annex sync` update the file mappings in direct mode.
To do this efficiently, it uses `git diff-tree` to find files that are
changed by the sync, and only updates those mappings. I'm rather happy
with this, as a first step to fully supporting sync in direct mode.

Finished the overhaul of the OSX app's library handling. It seems to work
well, and will fix a whole class of ways the OSX app could break.

Fixed a bug in the preferred content settings for backup repositories,
introduced by some changes I made to preferred content handling 4 days ago.

Fixed the Debian package to build with WebDAV support, which I forgot to
turn on before.

Planning a release tomorrow.
