Fix fork MRs importing issue

lib/gitlab/import_export/import_export.yml

@@ -133,5 +133,7 @@ methods:
     - :utf8_diff
   merge_requests:
     - :diff_head_sha
+    - :source_branch_sha
+    - :target_branch_sha
   project:
     - :description_html

lib/gitlab/import_export/merge_request_parser.rb

@@ -30,7 +30,7 @@ module Gitlab
       end
 
       def branch_exists?(branch_name)
-        @project.repository.branch_exists?(branch_name)
+        @project.repository.raw.branch_exists?(branch_name)
       end
 
       def fork_merge_request?

spec/lib/gitlab/import_export/project.json

@@ -2522,7 +2522,7 @@
       "id": 27,
       "target_branch": "feature",
       "source_branch": "feature_conflict",
-      "source_project_id": 5,
+      "source_project_id": 999,
       "author_id": 1,
       "assignee_id": null,
       "title": "MR1",
@@ -2536,6 +2536,9 @@
       "position": 0,
       "updated_by_id": null,
       "merge_error": null,
+      "diff_head_sha": "HEAD",
+      "source_branch_sha": "ABCD",
+      "target_branch_sha": "DCBA",
       "merge_params": {
         "force_remove_source_branch": null
       },

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -10,6 +10,10 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
         @shared = Gitlab::ImportExport::Shared.new(relative_path: "", project_path: 'path')
         allow(@shared).to receive(:export_path).and_return('spec/lib/gitlab/import_export/')
         @project = create(:project, :builds_disabled, :issues_disabled, name: 'project', path: 'project')
+
+        allow(@project.repository).to receive(:fetch_ref).and_return(true)
+        expect(@project.repository).to receive(:create_branch).with('feature', 'DCBA')
+
         project_tree_restorer = described_class.new(user: @user, shared: @shared, project: @project)
         @restored_project_json = project_tree_restorer.restore
       end

spec/lib/gitlab/import_export/project_tree_saver_spec.rb

@@ -11,6 +11,8 @@ describe Gitlab::ImportExport::ProjectTreeSaver do
     before do
       project.team << [user, :master]
       allow_any_instance_of(Gitlab::ImportExport).to receive(:storage_path).and_return(export_path)
+      allow_any_instance_of(MergeRequest).to receive(:source_branch_sha).and_return('ABCD')
+      allow_any_instance_of(MergeRequest).to receive(:target_branch_sha).and_return('DCBA')
     end
 
     after do
@@ -43,6 +45,14 @@ describe Gitlab::ImportExport::ProjectTreeSaver do
         expect(saved_project_json['merge_requests'].first['milestone']).not_to be_empty
       end
 
+      it 'has merge request\'s source branch SHA' do
+        expect(saved_project_json['merge_requests'].first['source_branch_sha']).to eq('ABCD')
+      end
+
+      it 'has merge request\'s target branch SHA' do
+        expect(saved_project_json['merge_requests'].first['target_branch_sha']).to eq('DCBA')
+      end
+
       it 'has events' do
         expect(saved_project_json['merge_requests'].first['milestone']['events']).not_to be_empty
       end