Track the previous default branch for a project when it changes

app/models/project.rb

@@ -416,6 +416,7 @@ class Project < ApplicationRecord
     prefix: :import, to: :import_state, allow_nil: true
   delegate :squash_always?, :squash_never?, :squash_enabled_by_default?, :squash_readonly?, to: :project_setting
   delegate :squash_option, to: :project_setting
+  delegate :previous_default_branch, :previous_default_branch=, to: :project_setting
   delegate :no_import?, to: :import_state, allow_nil: true
   delegate :name, to: :owner, allow_nil: true, prefix: true
   delegate :members, to: :team, prefix: true

app/services/projects/update_service.rb

@@ -66,6 +66,8 @@ module Projects
       previous_default_branch = project.default_branch
 
       if project.change_head(params[:default_branch])
+        params[:previous_default_branch] = previous_default_branch
+
         after_default_branch_change(previous_default_branch)
       else
         raise ValidationError, s_("UpdateProject|Could not set the default branch")

spec/services/projects/update_service_spec.rb

@@ -200,17 +200,32 @@ RSpec.describe Projects::UpdateService do
     context 'when updating a default branch' do
       let(:project) { create(:project, :repository) }
 
-      it 'changes a default branch' do
+      it 'changes default branch, tracking the previous branch' do
+        previous_default_branch = project.default_branch
+
         update_project(project, admin, default_branch: 'feature')
 
-        expect(Project.find(project.id).default_branch).to eq 'feature'
+        project.reload
+
+        expect(project.default_branch).to eq('feature')
+        expect(project.previous_default_branch).to eq(previous_default_branch)
+
+        update_project(project, admin, default_branch: previous_default_branch)
+
+        project.reload
+
+        expect(project.default_branch).to eq(previous_default_branch)
+        expect(project.previous_default_branch).to eq('feature')
       end
 
       it 'does not change a default branch' do
         # The branch 'unexisted-branch' does not exist.
         update_project(project, admin, default_branch: 'unexisted-branch')
 
-        expect(Project.find(project.id).default_branch).to eq 'master'
+        project.reload
+
+        expect(project.default_branch).to eq 'master'
+        expect(project.previous_default_branch).to be_nil
       end
     end