Fetch MR change using API

This enables rspec foss-impact job to run
in community and security forks

.gitlab/ci/rails.gitlab-ci.yml

@@ -331,11 +331,10 @@ rspec foss-impact:
   extends:
     - .rspec-base
     - .as-if-foss
-    - .rails:rules:ee-canonical-mr-only
+    - .rails:rules:ee-mr-only
     - .use-pg11
   script:
-    - git fetch origin $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
-    - git fetch origin $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME
+    - install_gitlab_gem
     - run_timed_command "scripts/gitaly-test-build"
     - run_timed_command "scripts/gitaly-test-spawn"
     - source scripts/rspec_helpers.sh

.gitlab/ci/rules.gitlab-ci.yml

@@ -446,13 +446,13 @@
     - <<: *if-master-refs
       changes: *code-backstage-patterns
 
-.rails:rules:ee-canonical-mr-only:
+.rails:rules:ee-mr-only:
   rules:
     - <<: *if-not-ee
       when: never
-    - <<: *if-not-canonical-namespace
-      when: never
-    - <<: *if-merge-request
+    - <<: *if-security-merge-request
+      changes: *code-backstage-patterns
+    - <<: *if-dot-com-gitlab-org-merge-request
       changes: *code-backstage-patterns
 
 .rails:rules:downtime_check:

tooling/bin/find_foss_tests

@@ -4,20 +4,22 @@
 require_relative '../../lib/gitlab/popen'
 require_relative '../lib/tooling/test_file_finder'
 
-output_file = ARGV.shift
-
-source = ENV.fetch('CI_MERGE_REQUEST_SOURCE_BRANCH_NAME')
-target = ENV.fetch('CI_MERGE_REQUEST_TARGET_BRANCH_NAME')
+require 'gitlab'
 
-git_diff = Gitlab::Popen.popen_with_detail(%w[git diff --name-only] + ["origin/#{target}...origin/#{source}"])
+gitlab_token = ENV.fetch('DANGER_GITLAB_API_TOKEN', '')
 
-unless git_diff.status.success?
-  warn "**** #{git_diff.cmd.join(' ')} had the following error(s):"
-  warn git_diff.stderr
-  exit git_diff.status.exitstatus
+Gitlab.configure do |config|
+  config.endpoint       = 'https://gitlab.com/api/v4'
+  config.private_token  = gitlab_token
 end
 
-changed_files = git_diff.stdout.split("\n")
+output_file = ARGV.shift
+
+mr_project_path = ENV.fetch('CI_MERGE_REQUEST_PROJECT_PATH')
+mr_iid = ENV.fetch('CI_MERGE_REQUEST_IID')
+
+mr_changes = Gitlab.merge_request_changes(mr_project_path, mr_iid)
+changed_files = mr_changes.changes.map { |change| change['new_path'] }
 
 tests_to_run = changed_files.flat_map do |file|
   test_files = Tooling::TestFileFinder.new(file, foss_test_only: true).test_files