GitHub import: Create new branches as project owner

This avoids permission errors when importing merge requests whose
authors don't have direct access to push to the main repository.

changelogs/unreleased/sh-create-branch-as-project-owner-for-github-import.yml

+---
+title: 'GitHub import: Create new branches as project owner'
+merge_request: 26335
+author:
+type: fixed

lib/gitlab/github_import/importer/pull_request_importer.rb

@@ -89,7 +89,7 @@ module Gitlab
 
           return if project.repository.branch_exists?(source_branch)
 
-          project.repository.add_branch(merge_request.author, source_branch, pull_request.source_branch_sha)
+          project.repository.add_branch(project.owner, source_branch, pull_request.source_branch_sha)
         rescue Gitlab::Git::CommandError => e
           Gitlab::Sentry.track_acceptable_exception(e,
                                                     extra: {

spec/lib/gitlab/github_import/importer/pull_request_importer_spec.rb

@@ -273,6 +273,11 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
       mr.state = 'opened'
       mr.save
 
+      # Ensure the project owner is creating the branches because the
+      # merge request author may not have access to push to this
+      # repository.
+      allow(project.repository).to receive(:add_branch).with(project.owner, anything, anything).and_call_original
+
       importer.insert_git_data(mr, exists)
 
       expect(project.repository.branch_exists?(mr.source_branch)).to be_truthy