Made `git annex export --to remote` honor the preferred content of the
remote. In a nice bit of code reuse, `adjustTree` was just what I needed to
filter unwanted content out of the exported tree.

Then a hard problem: When a tree is exported with some non-preferred
content filtered out, importing from the remote generates a tree that is
lacking those files, but merging that tree would delete the files
from the working tree. Solving that took the rest of the day.
