Fix creation of merge requests for orphaned branches

Closes #14875

CHANGELOG

@@ -15,6 +15,7 @@ v 8.7.0 (unreleased)
   - Implement 'Groups View' as an option for dashboard preferences !3379 (Elias W.)
   - Implement 'TODOs View' as an option for dashboard preferences !3379 (Elias W.)
   - Gracefully handle notes on deleted commits in merge requests (Stan Hu)
+  - Fix creation of merge requests for orphaned branches (Stan Hu)
   - Fall back to `In-Reply-To` and `References` headers when sub-addressing is not available (David Padilla)
   - Remove "Congratulations!" tweet button on newly-created project. (Connor Shea)
 

app/views/projects/diffs/_image.html.haml

 - diff = diff_file.diff
 - file_raw_path = namespace_project_raw_path(@project.namespace, @project, tree_join(@commit.id, diff.new_path))
-- old_commit_id = diff_refs.first.id
-- old_file_raw_path = namespace_project_raw_path(@project.namespace, @project, tree_join(old_commit_id, diff.old_path))
+// diff_refs will be nil for orphaned commits (e.g. first commit in repo)
+- if diff_refs
+  - old_commit_id = diff_refs.first.id
+  - old_file_raw_path = namespace_project_raw_path(@project.namespace, @project, tree_join(old_commit_id, diff.old_path))
+
 - if diff.renamed_file || diff.new_file || diff.deleted_file
   .image
     %span.wrap

spec/features/merge_requests/create_new_mr_spec.rb

+require 'spec_helper'
+
+feature 'Create New Merge Request', feature: true, js: false do
+  let(:user) { create(:user) }
+  let(:project) { create(:project, :public) }
+
+  before do
+    project.team << [user, :master]
+
+    login_as user
+    visit namespace_project_merge_requests_path(project.namespace, project)
+  end
+
+  it 'generates a diff for an orphaned branch' do
+    click_link 'New Merge Request'
+    select "orphaned-branch", from: "merge_request_source_branch"
+    select "master", from: "merge_request_target_branch"
+    click_button "Compare branches"
+
+    expect(page).to have_content "README.md"
+    expect(page).to have_content "wm.png"
+
+    fill_in "merge_request_title", with: "Orphaned MR test"
+    click_button "Submit merge request"
+
+    expect(page).to have_content 'git checkout -b orphaned-branch origin/orphaned-branch'
+  end
+end

spec/support/test_env.rb

@@ -15,6 +15,7 @@ module TestEnv
     'lfs'              => 'be93687',
     'master'           => '5937ac0',
     "'test'"           => 'e56497b',
+    'orphaned-branch'  => '45127a9',
   }
 
   # gitlab-test-fork is a fork of gitlab-fork, but we don't necessarily