`git annex export` normally exports all files in the specified tree,
which is generally what the user wants.
But, in some situations, the user may want to export a subset of files,
in a way that can be well expressed by a preferred content expression.

For example, they may want to export .mp3 files but not the .wav
files used to produce those.

Or, export podcasts, but not ones in a "old" directory that have already
been listened to.

It seems doable to make `git annex export` honor whatever
preferred content settings have been configured for the remote.
(And `git annex sync --content` too.)

> `git annex import` of a tree from a special remote would also be
> influenced by this.
> 
> It would make sense for the ImportableContents to have files
> that are not preferred content filtered out of it. Eg, if a .wav file
> is added to the remote, it shouldn't be downloaded. Or a better example,
> if directory Music is excluded from an android remote, importing from
> it should exclude that directory.

> Problem: If a tree is exported with eg, no .wav files, and then an import
> is made from the remote, and necessarily lacks .wav files, the remote
> tracking branch will have a tree with no .wav
> files. Merging that into master will delete all the .wav files.
> 
> If the remote tracking branch has a disconnected history from master,
> then git wouldn't delete files on
> merge. But: This would prevent actual deletions made on the special
> remote from happening in master too. So not a good idea.
> 
> So it seems that, when updating the remote tracking branch, the files
> that were excluded from being exported to it need to be added back in.
> That can be done by keeping a pointer to the tree that was last exported, 
> including all exported files, and generating a tree from it that deletes
> all exported files. The diff between those trees is all the excluded
> files, so when updating the remote tracking branch, take the tree that
> was imported, and merge that diff into it.