Make the first branch pushed to an empty repository the default HEAD.

In an empty repository, pushing a new branch not called "master" would
leave HEAD in an unknown state, causing ambiguity if another branch
were pushed. This could in turn cause a new protected branch to be
created and cause the default branch to change.

* Closes #1561
* Closes #1576
* Closes https://github.com/gitlabhq/gitlabhq/issues/8883

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.11.0 (unreleased)
+  - Make the first branch pushed to an empty repository the default HEAD (Stan Hu)
   - Make Reply-To config apply to change e-mail confirmation and other Devise notifications (Stan Hu)
   - Add application setting to restrict user signups to e-mail domains (Stan Hu)
   - Don't allow a merge request to be merged when its title starts with "WIP".

app/services/git_push_service.rb

@@ -31,6 +31,10 @@ class GitPushService
         # Initial push to the default branch. Take the full history of that branch as "newly pushed".
         @push_commits = project.repository.commits(newrev)
 
+        # Ensure HEAD points to the default branch in case it is not master
+        branch_name = Gitlab::Git.ref_name(ref)
+        project.change_head(branch_name)
+
         # Set protection on the default branch if configured
         if (current_application_settings.default_branch_protection != PROTECTION_NONE)
           developers_can_push = current_application_settings.default_branch_protection == PROTECTION_DEV_CAN_PUSH ? true : false

spec/services/git_push_service_spec.rb

@@ -234,5 +234,18 @@ describe GitPushService do
       expect(Issue.find(issue.id)).to be_opened
     end
   end
-end
 
+  describe "empty project" do
+    let(:project) { create(:project_empty_repo) }
+    let(:new_ref) { 'refs/heads/feature'}
+
+    before do
+      allow(project).to receive(:default_branch).and_return('feature')
+      expect(project).to receive(:change_head) { 'feature'}
+    end
+
+    it 'push to first branch updates HEAD' do
+      service.execute(project, user, @blankrev, @newrev, new_ref)
+    end
+  end
+end