Fix blob preview error

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/330737

**Problem**

Diff lines are not converted to an array. This causes an error.

**Solution**

Convert diff lines to the array

Changelog: fixed

app/controllers/projects/blob_controller.rb

@@ -92,7 +92,7 @@ class Projects::BlobController < Projects::ApplicationController
     @blob.load_all_data!
     diffy = Diffy::Diff.new(@blob.data, @content, diff: '-U 3', include_diff_info: true)
     diff_lines = diffy.diff.scan(/.*\n/)[2..-1]
-    diff_lines = Gitlab::Diff::Parser.new.parse(diff_lines)
+    diff_lines = Gitlab::Diff::Parser.new.parse(diff_lines).to_a
     @diff_lines = Gitlab::Diff::Highlight.new(diff_lines, repository: @repository).highlight
 
     render layout: false

changelogs/unreleased/330737_fix_blob_preview.yml

+---
+title: Fix blob preview error
+merge_request: 62128
+author:
+type: fixed

spec/controllers/projects/blob_controller_spec.rb

@@ -444,6 +444,40 @@ RSpec.describe Projects::BlobController do
     end
   end
 
+  describe 'POST preview' do
+    subject(:request) { post :preview, params: default_params }
+
+    let(:user) { create(:user) }
+    let(:filename) { 'preview.md' }
+    let(:default_params) do
+      {
+        namespace_id: project.namespace,
+        project_id: project,
+        id: "#{project.default_branch}/#{filename}",
+        content: "Bar\n"
+      }
+    end
+
+    before do
+      project.add_developer(user)
+      sign_in(user)
+
+      project.repository.create_file(
+        project.creator,
+        filename,
+        "Foo\n",
+        message: 'Test',
+        branch_name: project.default_branch
+      )
+    end
+
+    it 'is successful' do
+      request
+
+      expect(response).to be_successful
+    end
+  end
+
   describe 'POST create' do
     let(:user) { create(:user) }
     let(:default_params) do