BURs: when moving artists, add old tag to other names.

When aliasing or renaming an artist, add their old tag name to their
other names.

app/logical/tag_mover.rb

@@ -35,7 +35,9 @@ class TagMover
     return unless old_tag.artist? && old_artist.present? && !old_artist.is_deleted?
 
     if new_artist.nil?
-      old_artist.update!(name: new_tag.name)
+      old_artist.name = new_tag.name
+      old_artist.other_names += [old_tag.name]
+      old_artist.save!
     else
       merge_artists!
     end

test/unit/bulk_update_request_test.rb

@@ -285,6 +285,12 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
           assert_equal(["Can't rename aaa -> bbb ('aaa' has more than 200 posts, use an alias instead)"], @bur.errors.full_messages)
         end
 
+        context "when moving an artist" do
+          should "add the artist's old tag name to their other names" do
+            assert_equal(["foo"], @artist.reload.other_names)
+          end
+        end
+
         context "when renaming a character tag with a *_(cosplay) tag" do
           should "move the *_(cosplay) tag as well" do
             @post = create(:post, tag_string: "toosaka_rin_(cosplay)")