Fix arguments error on Import/Export fetch_ref method

Added unit test and updated integration spec to test for this as well.

lib/gitlab/import_export/merge_request_parser.rb

@@ -26,7 +26,7 @@ module Gitlab
       end
 
       def fetch_ref
-        @project.repository.fetch_ref(@project.repository.path, @diff_head_sha, @merge_request.source_branch)
+        @project.repository.fetch_ref(@project.repository.path, source_ref: @diff_head_sha, target_ref: @merge_request.source_branch)
       end
 
       def branch_exists?(branch_name)

spec/lib/gitlab/import_export/merge_request_parser_spec.rb

@@ -13,7 +13,7 @@ describe Gitlab::ImportExport::MergeRequestParser do
 
   let(:parsed_merge_request) do
     described_class.new(project,
-                        merge_request.diff_head_sha,
+                        'abcd',
                         merge_request,
                         merge_request.as_json).parse!
   end
@@ -29,4 +29,14 @@ describe Gitlab::ImportExport::MergeRequestParser do
   it 'has a target branch' do
     expect(project.repository.branch_exists?(parsed_merge_request.target_branch)).to be true
   end
+
+  it 'parses a MR that has no source branch' do
+    allow_any_instance_of(Gitlab::ImportExport::MergeRequestParser).to receive(:branch_exists?).and_call_original
+    allow_any_instance_of(Gitlab::ImportExport::MergeRequestParser).to receive(:branch_exists?).with(merge_request.source_branch).and_return(false)
+    allow_any_instance_of(Gitlab::ImportExport::MergeRequestParser).to receive(:fork_merge_request?).and_return(true)
+    allow(Gitlab::GitalyClient).to receive(:migrate).and_call_original
+    allow(Gitlab::GitalyClient).to receive(:migrate).with(:fetch_ref).and_return([nil, 0])
+
+    expect(parsed_merge_request).to eq(merge_request)
+  end
 end