Fix NoMethodError for CommitController

* Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/351520
* Sentry error:
https://sentry.gitlab.net/gitlab/gitlabcom/issues/3214315

Changelog: fixed

app/controllers/projects/commit_controller.rb

@@ -106,6 +106,8 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def revert
+    return render_404 unless @commit
+
     assign_change_commit_vars
 
     return render_404 if @start_branch.blank?
@@ -117,6 +119,8 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def cherry_pick
+    return render_404 unless @commit
+
     assign_change_commit_vars
 
     return render_404 if @start_branch.blank?

spec/controllers/projects/commit_controller_spec.rb

@@ -212,6 +212,21 @@ RSpec.describe Projects::CommitController do
       end
     end
 
+    context 'when the revert commit is missing' do
+      it 'renders the 404 page' do
+        post(:revert,
+            params: {
+              namespace_id: project.namespace,
+              project_id: project,
+              start_branch: 'master',
+              id: '1234567890'
+            })
+
+        expect(response).not_to be_successful
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
     context 'when the revert was successful' do
       it 'redirects to the commits page' do
         post(:revert,
@@ -269,6 +284,21 @@ RSpec.describe Projects::CommitController do
       end
     end
 
+    context 'when the cherry-pick commit is missing' do
+      it 'renders the 404 page' do
+        post(:cherry_pick,
+            params: {
+              namespace_id: project.namespace,
+              project_id: project,
+              start_branch: 'master',
+              id: '1234567890'
+            })
+
+        expect(response).not_to be_successful
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
     context 'when the cherry-pick was successful' do
       it 'redirects to the commits page' do
         post(:cherry_pick,