Fix various issues with account migration (#12301)

* Fix being able to follow oneself by moving to an account that was following the old one

* Add specs

* Add spec to catch MoveWorker issue with local followers following both accounts

* Fix move worker breaking when a local account follows both source and target accounts

* Fix migration from remote to local account not sending Undo Follow

* Fix show_reblogs not being preserved for moved account's followers

spec/workers/unfollow_follow_worker_spec.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe UnfollowFollowWorker do
+  let(:local_follower)   { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob')).account }
+  let(:source_account)   { Fabricate(:account) }
+  let(:target_account)   { Fabricate(:account) }
+  let(:show_reblogs)     { true }
+
+  subject { described_class.new }
+
+  before do
+    local_follower.follow!(source_account, reblogs: show_reblogs)
+  end
+
+  context 'when show_reblogs is true' do
+    let(:show_reblogs) { true }
+
+    describe 'perform' do
+      it 'unfollows source account and follows target account' do
+        subject.perform(local_follower.id, source_account.id, target_account.id)
+        expect(local_follower.following?(source_account)).to be false
+        expect(local_follower.following?(target_account)).to be true
+      end
+
+      it 'preserves show_reblogs' do
+        subject.perform(local_follower.id, source_account.id, target_account.id)
+        expect(Follow.find_by(account: local_follower, target_account: target_account).show_reblogs?).to be show_reblogs
+      end
+    end
+  end
+
+  context 'when show_reblogs is false' do
+    let(:show_reblogs) { false }
+
+    describe 'perform' do
+      it 'unfollows source account and follows target account' do
+        subject.perform(local_follower.id, source_account.id, target_account.id)
+        expect(local_follower.following?(source_account)).to be false
+        expect(local_follower.following?(target_account)).to be true
+      end
+
+      it 'preserves show_reblogs' do
+        subject.perform(local_follower.id, source_account.id, target_account.id)
+        expect(Follow.find_by(account: local_follower, target_account: target_account).show_reblogs?).to be show_reblogs
+      end
+    end
+  end
+end