I've developed a plan for how to handle [[todo/export_preferred_content]].
And today I'm working on making `git annex import --from remote` honor
the preferred content of the remote. It doesn't make sense to support it
for one and not the other, so this is on the `preferred` git branch for now.

One use case for this is to configure an import to exclude certain file
extensions or directories. Such unwanted content will be left as-is 
in the remote's data store, but won't be imported, so from git-annex's
POV, it won't be present on the remote.

The tricky thing is, when importing, the key is not known until the file
is downloaded, but you don't want git-annex downloading content that is not
preferred. I'm finessing that problem by checking the subset of preferred
content expressions that are not dependent on the file's content, which will
avoid downloads of unwanted content in probably most cases.

What should it do when the preferred content expression is dependent on
the file's content? I'm undecided if it's better to warn and not import,
or to download the content once in order to check the preferred content
expression, and then throw unwanted content away.
