GitHub importer: Use the project creator to create branches from forks

The project owner may actually be a group, in which case the branch
creation would fail due to a lack of username.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59396

changelogs/unreleased/sh-fix-github-import-creator.yml

+---
+title: 'GitHub importer: Use the project creator to create branches from forks'
+merge_request: 26510
+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(project.owner, source_branch, pull_request.source_branch_sha)
+          project.repository.add_branch(project.creator, 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

@@ -273,10 +273,10 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
       mr.state = 'opened'
       mr.save
 
-      # Ensure the project owner is creating the branches because the
+      # Ensure the project creator 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
+      # repository. The project owner may also be a group.
+      allow(project.repository).to receive(:add_branch).with(project.creator, anything, anything).and_call_original
 
       importer.insert_git_data(mr, exists)