Fix 'from_scratch': delete all tags before writing new tags to file

## Github Issues
Fixes 	#3706
Related #5165

## Issue
Comment tags are written to file even if option 'from_scratch' is used.
The same tags are not written to the file if imported together with other files as album.
Therefore 'from_scratch' is not working as described in the documentation.

## Solution
1. Add test: Adapt the function from the 'regular' import class and insert it in the class for the singleton import test.
2. Fix bug : Add check for 'from_scratch' option. If used, clear metadata before applying 'new' metadata with autotag.
3. No documentation change needed. Option now works as described in the documentation.
4. Add changelog.

beets/importer/tasks.py

@@ -690,6 +690,8 @@ class SingletonImportTask(ImportTask):
         return [self.item]
 
     def apply_metadata(self):
+        if config["import"]["from_scratch"]:
+            self.item.clear()
         autotag.apply_item_metadata(self.item, self.match.info)
 
     def _emit_imported(self, lib):

docs/changelog.rst

@@ -39,6 +39,9 @@ Bug fixes:
   :bug:`5797`
 * :doc:`plugins/musicbrainz`: Fix the MusicBrainz search not taking into
   account the album/recording aliases
+* :doc:`reference/cli`: Fix 'from_scratch' option for singleton imports: delete
+  all (old) metadata when new metadata is applied.
+  :bug:`3706`
 
 For packagers:
 

test/test_importer.py

@@ -315,6 +315,17 @@ class ImportSingletonTest(AutotagImportTestCase):
         self.importer.run()
         self.assert_file_in_lib(b"singletons", b"Applied Track 1.mp3")
 
+    def test_apply_from_scratch_removes_other_metadata(self):
+        config["import"]["from_scratch"] = True
+
+        for mediafile in self.import_media:
+            mediafile.comments = "Tag Comment"
+            mediafile.save()
+
+        self.importer.add_choice(importer.Action.APPLY)
+        self.importer.run()
+        assert self.lib.items().get().comments == ""
+
     def test_skip_does_not_add_first_track(self):
         self.importer.add_choice(importer.Action.SKIP)
         self.importer.run()