Speed up diff comparisons by limiting number of commit messages rendered

When a diff has a significant number of commits, the previous behavior would
attempt to render the Markdown on all the commit messages but only display
1000 of them. To avoid additional work, we only need to render the Markdown
on the set that is displayed.

2 files changed, 28 insertions, 0 deletions

diff --git a/spec/views/projects/merge_requests/_commits.html.haml_spec.rb b/spec/views/projects/merge_requests/_commits.html.haml_spec.rb
index b1c6565c08a..a7628548de6 100644
--- a/spec/views/projects/merge_requests/_commits.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/_commits.html.haml_spec.rb
@@ -20,6 +20,7 @@ describe 'projects/merge_requests/_commits.html.haml' do
 
     assign(:merge_request, merge_request)
     assign(:commits, merge_request.commits)
+    assign(:hidden_commit_count, 0)
   end
 
   it 'shows commits from source project' do
@@ -30,4 +31,16 @@ describe 'projects/merge_requests/_commits.html.haml' do
 
     expect(rendered).to have_link(href: href)
   end
+
+  context 'when there are hidden commits' do
+    before do
+      assign(:hidden_commit_count, 1)
+    end
+
+    it 'shows notice about omitted commits' do
+      render
+
+      expect(rendered).to match(/1 additional commit has been omitted to prevent performance issues/)
+    end
+  end
 end
diff --git a/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb b/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb
index 9ab105c3238..8befae39d3a 100644
--- a/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb
@@ -9,6 +9,8 @@ describe 'projects/merge_requests/creations/_new_submit.html.haml' do
 
     assign(:merge_request, merge_request)
     assign(:commits, merge_request.commits)
+    assign(:hidden_commit_count, 0)
+    assign(:total_commit_count, merge_request.commits.count)
     assign(:project, merge_request.target_project)
 
     allow(view).to receive(:can?).and_return(true)
@@ -29,4 +31,17 @@ describe 'projects/merge_requests/creations/_new_submit.html.haml' do
       expect(rendered).not_to have_text('Builds')
     end
   end
+
+  context 'when there are hidden commits' do
+    before do
+      assign(:pipelines, Ci::Pipeline.none)
+      assign(:hidden_commit_count, 2)
+    end
+
+    it 'shows notice about omitted commits' do
+      render
+
+      expect(rendered).to match(/2 additional commits have been omitted to prevent performance issues/)
+    end
+  end
 end