Merge branch 'sh-fix-root-ref-repository' into 'master'

changelogs/unreleased/sh-fix-root-ref-repository.yml

+---
+title: "Gracefully handle case when repository's root ref does not exist"
+merge_request:
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -1222,6 +1222,8 @@ module Gitlab
       end
 
       def git_merged_branch_names(branch_names = [])
+        return [] unless root_ref
+
         lines = run_git(['branch', '--merged', root_ref] + branch_names)
           .first.lines
 

spec/lib/gitlab/git/repository_spec.rb

@@ -1217,6 +1217,16 @@ describe Gitlab::Git::Repository, seed_helper: true do
       end
     end
 
+    context 'when no root ref is available' do
+      it 'returns empty list' do
+        project = create(:project, :empty_repo)
+
+        names = project.repository.merged_branch_names(%w[feature])
+
+        expect(names).to be_empty
+      end
+    end
+
     context 'when no branch names are specified' do
       it 'returns all merged branch names' do
         names = repository.merged_branch_names