Use create_branch instead of fetch_source_branch when importing MRs

When importing, we want to create `refs/heads/:source_branch` in the
repository for each MR. Since the imported project already contains
the SHA, there's no need to use `fetch_source_branch` - we can just
create the ref directly.

lib/gitlab/import_export/merge_request_parser.rb

@@ -16,7 +16,7 @@ module Gitlab
         if fork_merge_request? && @diff_head_sha
           @merge_request.source_project_id = @relation_hash['project_id']
 
-          fetch_ref unless branch_exists?(@merge_request.source_branch)
+          create_source_branch unless branch_exists?(@merge_request.source_branch)
           create_target_branch unless branch_exists?(@merge_request.target_branch)
         end
 
@@ -34,17 +34,18 @@ module Gitlab
         @merge_request
       end
 
-      def create_target_branch
-        @project.repository.create_branch(@merge_request.target_branch, @merge_request.target_branch_sha)
+      # When the exported MR was in a fork, the source branch does not exist in
+      # the imported bundle - although the commits usually do - so it must be
+      # created manually. Ignore failures so we get the merge request itself if
+      # the commits are missing.
+      def create_source_branch
+        @project.repository.create_branch(@merge_request.source_branch, @diff_head_sha)
+      rescue => err
+        Rails.logger.warn("Import/Export warning: Failed to create source branch #{@merge_request.source_branch} => #{@diff_head_sha} for MR #{@merge_request.iid}: #{err}") # rubocop:disable Gitlab/RailsLogger
       end
 
-      # Gitaly migration: https://gitlab.com/gitlab-org/gitaly/issues/1295
-      def fetch_ref
-        target_ref = Gitlab::Git::BRANCH_REF_PREFIX + @merge_request.source_branch
-
-        unless @project.repository.fetch_source_branch!(@project.repository, @diff_head_sha, target_ref)
-          Rails.logger.warn("Import/Export warning: Failed to create #{target_ref} for MR: #{@merge_request.iid}") # rubocop:disable Gitlab/RailsLogger
-        end
+      def create_target_branch
+        @project.repository.create_branch(@merge_request.target_branch, @merge_request.target_branch_sha)
       end
 
       def branch_exists?(branch_name)

spec/lib/gitlab/import_export/merge_request_parser_spec.rb

@@ -40,8 +40,6 @@ describe Gitlab::ImportExport::MergeRequestParser do
       allow(instance).to receive(:branch_exists?).with(merge_request.source_branch).and_return(false)
       allow(instance).to receive(:fork_merge_request?).and_return(true)
     end
-    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