Fix wrong default branch imported with Bitbucket Server

When importing a repository from Bitbucket Server where HEAD of that
repo is something different than the default HEAD as created by Gitaly
(which currently is "main", so it would be triggered by importing any
repo whose HEAD points to "master"), then the end result is a corrupt
repository. This is because the import will create repos by first
calling CreateRepository, which initializes HEAD to "main", and then
calling FetchRemote, which only fetches refs but won't ever update HEAD.

We fix this by doing a standard import of the repository with a
follow-up fetch to map the pull requests to merge request references.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/346008

Changelog: fixed

lib/gitlab/bitbucket_server_import/importer.rb

@@ -19,7 +19,8 @@ module Gitlab
       end
 
       def self.refmap
-        [:heads, :tags, '+refs/pull-requests/*/to:refs/merge-requests/*/head']
+        # We omit :heads and :tags since these are fetched in the import_repository
+        ['+refs/pull-requests/*/to:refs/merge-requests/*/head']
       end
 
       # Unlike GitHub, you can't grab the commit SHAs for pull requests that
@@ -140,7 +141,7 @@ module Gitlab
       def import_repository
         log_info(stage: 'import_repository', message: 'starting import')
 
-        project.ensure_repository
+        project.repository.import_repository(project.import_url)
         project.repository.fetch_as_mirror(project.import_url, refmap: self.class.refmap)
 
         log_info(stage: 'import_repository', message: 'finished import')

spec/lib/gitlab/bitbucket_server_import/importer_spec.rb

@@ -27,12 +27,18 @@ RSpec.describe Gitlab::BitbucketServerImport::Importer do
   end
 
   describe '#import_repository' do
+    let(:repo_url) { 'http://bitbucket:test@my-bitbucket' }
+
+    before do
+      expect(project.repository).to receive(:import_repository).with(repo_url)
+    end
+
     it 'adds a remote' do
       expect(subject).to receive(:import_pull_requests)
       expect(subject).to receive(:delete_temp_branches)
       expect(project.repository).to receive(:fetch_as_mirror)
-                                     .with('http://bitbucket:test@my-bitbucket',
-                                           refmap: [:heads, :tags, '+refs/pull-requests/*/to:refs/merge-requests/*/head'])
+                                     .with(repo_url,
+                                           refmap: ['+refs/pull-requests/*/to:refs/merge-requests/*/head'])
 
       subject.execute
     end