Got merging working in direct mode!

Basically works as outlined yesterday, although slightly less clumsily.
Since there was already code that ran `git diff-tree` to update the
associated files mappings after a merge, I was able to adapt that same code
to also update the working tree.

An important invariant for direct mode merges is that they should never
cause annexed objects to be dropped. So if a file is deleted by a merge,
and was a direct mode file that was the only place in the working copy
where an object was stored, the object is moved into `.git/annex/objects`.
This avoids data loss and any need to re-transfer objects after a merge.
It also makes renames and other move complex tree manipulations always end
up with direct mode files, when their content was present.

Automatic merge conflict resoltion doesn't quite work right yet in direct
mode.

Direct mode has landed in the `master` branch, but I still consider it
experimental, and of course the assistant still needs to be updated to
support it.
