Disable commit checks when no push rules are active

The refactoring in 0a8db3dc
inadvertently caused commit checks to become active regardless of
whether any rules were actually active. This significantly slowed pushes
and caused repositories with large number of commits to time out.

We fix this by using `push_rule_checks_commit?` since the existence of a
`PushRule` is not sufficient to determine whether any rules are
active. This also eliminates the need to override
`should_run_diff_validations?`, since the set of validations will only
contain the active ones.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57067

ee/changelogs/unreleased/sh-fix-issue-57067.yml

+---
+title: Disable commit checks when no push rules are active
+merge_request: 9569
+author:
+type: performance

ee/lib/ee/gitlab/checks/diff_check.rb

@@ -9,11 +9,6 @@ module EE
 
         private
 
-        override :should_run_diff_validations?
-        def should_run_diff_validations?
-          super || validate_path_locks? || push_rule_checks_commit?
-        end
-
         def validate_path_locks?
           strong_memoize(:validate_path_locks) do
             project.feature_available?(:file_locks) &&
@@ -32,7 +27,7 @@ module EE
         def validations_for_diff
           super.tap do |validations|
             validations.push(path_locks_validation) if validate_path_locks?
-            validations.push(file_name_validation) if push_rule
+            validations.push(file_name_validation) if push_rule_checks_commit?
           end
         end
 

ee/spec/lib/gitlab/checks/diff_check_spec.rb

@@ -6,6 +6,16 @@ describe Gitlab::Checks::DiffCheck do
   include_context 'push rules checks context'
 
   describe '#validate!' do
+    context 'no push rules active' do
+      set(:push_rule) { create(:push_rule) }
+
+      it "does not attempt to check commits" do
+        expect(subject).not_to receive(:process_commits)
+
+        subject.validate!
+      end
+    end
+
     context 'file name rules' do
       # Notice that the commit used creates a file named 'README'
       context 'file name regex check' do