Fix reflink: "auto" option

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.

beets/importer.py

@@ -1684,6 +1684,8 @@ def manipulate_files(session, task):
             operation = MoveOperation.LINK
         elif session.config["hardlink"]:
             operation = MoveOperation.HARDLINK
+        elif session.config["reflink"] == "auto":
+            operation = MoveOperation.REFLINK_AUTO
         elif session.config["reflink"]:
             operation = MoveOperation.REFLINK
         else:

docs/changelog.rst

@@ -9,6 +9,7 @@ Changelog goes here! Please add your entry to the bottom of one of the lists bel
 Bug fixes:
 
 * Improved naming of temporary files by separating the random part with the file extension.
+* Fixed the ``auto`` value for the :ref:`reflink` config option.
 
 For packagers:
 

test/test_importer.py

@@ -235,6 +235,30 @@ def test_import_hardlink_arrives(self):
                 == (s2[stat.ST_INO], s2[stat.ST_DEV])
             )
 
+    @unittest.skipUnless(_common.HAVE_REFLINK, "need reflinks")
+    def test_import_reflink_arrives(self):
+        # Detecting reflinks is currently tricky due to various fs
+        # implementations, we'll just check the file exists.
+        config["import"]["reflink"] = True
+        self.importer.run()
+        for mediafile in self.import_media:
+            self.assert_file_in_lib(
+                b"Tag Artist",
+                b"Tag Album",
+                util.bytestring_path(f"{mediafile.title}.mp3"),
+            )
+
+    def test_import_reflink_auto_arrives(self):
+        # Should pass regardless of reflink support due to fallback.
+        config["import"]["reflink"] = "auto"
+        self.importer.run()
+        for mediafile in self.import_media:
+            self.assert_file_in_lib(
+                b"Tag Artist",
+                b"Tag Album",
+                util.bytestring_path(f"{mediafile.title}.mp3"),
+            )
+
 
 def create_archive(session):
     (handle, path) = mkstemp(dir=py3_path(session.temp_dir))