Fix #4829: DanbooruBot not properly handling BUR aliases when target tag has a wiki.

app/logical/tag_mover.rb

@@ -151,13 +151,16 @@ class TagMover
     old_artist.save!
   end
 
-  # Merge the other names from both wikis, then mark the old wiki as deleted.
+  # Merge the other names from both wikis. Transfer the body from the old wiki
+  # to the new wiki if the new wiki has an empty body. Then mark the old wiki
+  # as deleted.
   def merge_wikis!
     old_wiki.lock!
     new_wiki.lock!
 
     new_wiki.other_names += old_wiki.other_names
     new_wiki.is_deleted = false
+    new_wiki.body = old_wiki.body if new_wiki.body.blank? && old_wiki.body.present?
     new_wiki.save!
 
     old_wiki.body = "This tag has been moved to [[#{new_wiki.title}]]."

test/unit/tag_alias_test.rb

@@ -180,6 +180,17 @@ class TagAliasTest < ActiveSupport::TestCase
         assert_equal("second", @wiki2.body)
       end
 
+      should "move the old wiki body to the new wiki if the new wiki has a blank body" do
+        @wiki1 = create(:wiki_page, title: "aaa", other_names: "111 222", body: "first")
+        @wiki2 = create(:wiki_page, title: "bbb", other_names: "111 333", body: "")
+
+        TagAlias.approve!(antecedent_name: "aaa", consequent_name: "bbb", approver: @admin)
+        perform_enqueued_jobs
+
+        assert_equal("This tag has been moved to [[#{@wiki2.title}]].", @wiki1.reload.body)
+        assert_equal("first", @wiki2.reload.body)
+      end
+
       should "ignore the old wiki if it has been deleted" do
         @wiki1 = create(:wiki_page, title: "aaa", other_names: "111 222", body: "first", is_deleted: true)
         @wiki2 = create(:wiki_page, title: "bbb", other_names: "111 333", body: "second")