Apply fix from @jacobvosmaer-gitlab.

See broken build here: https://gitlab.com/gitlab-org/gitlab-ee/builds/3763020

app/models/repository.rb

@@ -1247,7 +1247,11 @@ class Repository
       raise CommitError.new('Failed to create commit')
     end
 
-    oldrev = rugged.lookup(newrev).parent_ids.first || Gitlab::Git::BLANK_SHA
+    if rugged.lookup(newrev).parent_ids.empty? || target_branch.nil?
+      oldrev = Gitlab::Git::BLANK_SHA
+    else
+      oldrev = rugged.merge_base(newrev, target_branch.target.sha)
+    end
 
     GitHooksService.new.execute(current_user, path_to_repo, oldrev, newrev, ref) do
       update_ref!(ref, newrev, oldrev)

spec/models/repository_spec.rb

@@ -750,32 +750,10 @@ describe Repository, models: true do
   end
 
   describe '#ff_merge' do
-    it 'merges the code and return the commit id' do
-      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'master', source_project: project)
-      merge_commit_id = repository.ff_merge(user,
-                                            merge_request.diff_head_sha,
-                                            merge_request.target_branch,
-                                            merge_request: merge_request)
-      merge_commit = repository.commit(merge_commit_id)
+    before { repository.add_branch(user, 'ff-target', 'feature~5') }
 
-      expect(merge_commit).to be_present
-      expect(repository.blob_at(merge_commit.id, 'files/ruby/feature.rb')).to be_present
-    end
-
-    it 'sets the `in_progress_merge_commit_sha` flag for the given merge request' do
-      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'master', source_project: project)
-      merge_commit_id = repository.ff_merge(user,
-                                            merge_request.diff_head_sha,
-                                            merge_request.target_branch,
-                                            merge_request: merge_request)
-
-      expect(merge_request.in_progress_merge_commit_sha).to eq(merge_commit_id)
-    end
-  end
-
-  describe '#ff_merge' do
     it 'merges the code and return the commit id' do
-      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'master', source_project: project)
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'ff-target', source_project: project)
       merge_commit_id = repository.ff_merge(user,
                                             merge_request.diff_head_sha,
                                             merge_request.target_branch,
@@ -787,7 +765,7 @@ describe Repository, models: true do
     end
 
     it 'sets the `in_progress_merge_commit_sha` flag for the given merge request' do
-      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'master', source_project: project)
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'ff-target', source_project: project)
       merge_commit_id = repository.ff_merge(user,
                                             merge_request.diff_head_sha,
                                             merge_request.target_branch,