Fix repository ff_merge method

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/repository.rb

@@ -864,7 +864,7 @@ class Repository
     raise 'Invalid merge target' if our_commit.nil?
     raise 'Invalid merge source' if their_commit.nil?
 
-    Gitlab::Git::OperationService.new(user, raw_repository).with_branch(target_branch) do |start_commit|
+    with_branch(user, target_branch) do |start_commit|
       merge_request&.update(in_progress_merge_commit_sha: their_commit.oid)
 
       their_commit.oid

spec/ee/spec/models/ee/merge_request_spec.rb

@@ -38,6 +38,9 @@ describe MergeRequest do
   end
 
   describe '#rebase_in_progress?' do
+    # Create merge request and project before we stub file calls
+    before { subject }
+
     it 'returns true when there is a current rebase directory' do
       allow(File).to receive(:exist?).and_return(true)
       allow(File).to receive(:mtime).and_return(Time.now)
@@ -69,6 +72,9 @@ describe MergeRequest do
   end
 
   describe '#squash_in_progress?' do
+    # Create merge request and project before we stub file calls
+    before { subject }
+
     it 'returns true when there is a current squash directory' do
       allow(File).to receive(:exist?).and_return(true)
       allow(File).to receive(:mtime).and_return(Time.now)