Fix Spinach merge request diff failures

gitlab-git-test `master` was updated in ff076d88, and this caused
the merge request diffs to change in a way that broke assumptions in the
Spinach tests.

Partial fix to #23378

features/steps/project/merge_requests.rb

@@ -114,7 +114,7 @@ class Spinach::Features::ProjectMergeRequests < Spinach::FeatureSteps
            source_project: project,
            target_project: project,
            source_branch: 'fix',
-           target_branch: 'master',
+           target_branch: 'merge-test',
            author: project.users.first,
            description: "# Description header"
           )
@@ -137,7 +137,8 @@ class Spinach::Features::ProjectMergeRequests < Spinach::FeatureSteps
            title: "Bug NS-05",
            source_project: project,
            target_project: project,
-           author: project.users.first)
+           author: project.users.first,
+           source_branch: 'merge-test')
   end
   step 'project "Shop" have "Feature NS-05" merged merge request' do
@@ -508,7 +509,7 @@ class Spinach::Features::ProjectMergeRequests < Spinach::FeatureSteps
   step 'I should see new target branch changes' do
     expect(page).to have_content 'Request to merge fix into feature'
-    expect(page).to have_content 'Target branch changed from master to feature'
+    expect(page).to have_content 'Target branch changed from merge-test to feature'
   end
   step 'I click on "Email Patches"' do

spec/support/test_env.rb

@@ -17,6 +17,7 @@ module TestEnv
     'markdown'                           => '0ed8c6c',
     'lfs'                                => 'be93687',
     'master'                             => 'b83d6e3',
+    'merge-test'                         => '5937ac0',
     "'test'"                             => 'e56497b',
     'orphaned-branch'                    => '45127a9',
     'binary-encoding'                    => '7b1cf43',