Fix 'from_scratch': delete all tags before writing new tags to file (#5828)

Fixes #3706.

### 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

@@ -680,6 +680,8 @@ 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

@@ -90,6 +90,8 @@ Bug fixes:
 - :doc:`/plugins/ftintitle`: Fixed artist name splitting to prioritize explicit
   featuring tokens (feat, ft, featuring) over generic separators (&, and),
   preventing incorrect splits when both are present.
+- :doc:`reference/cli`: Fix 'from_scratch' option for singleton imports: delete
+  all (old) metadata when new metadata is applied. :bug:`3706`
 
 For plugin developers:
 

test/test_importer.py

@@ -258,6 +258,17 @@ class ImportSingletonTest(AutotagImportTestCase):
         assert self.lib.items().get().title == "Applied Track 1"
         assert (self.lib_path / "singletons" / "Applied Track 1.mp3").exists()
 
+    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_track(self):
         self.importer.add_choice(importer.Action.SKIP)
         self.importer.run()