Fix `reflink: "auto"` option

Description

The docs say:

The auto option uses reflinks when possible and falls back to plain copying when necessary.

I've been using this option for a while, and recently discovered that despite the option, copying fails between two BTRFS filesystems with:

Error: OS/filesystem does not support reflinks. during link of paths /mnt/fs1/file, /mnt/fs2/file

I tracked this down to how the configuration is handled in the importer.

I love beets dearly, thanks for your great work :)

[x] Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
[x] Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
[x] Tests. (Very much encouraged but not strictly required.)