Generate correct ETags when sending Git blobs

Calling `#stale?` already generates and sets the response ETag so we
should not set it manually.

Setting it manually actually broke caching because Rails now has extra
validators that is combined with the `blob.id` that we provide.
https://github.com/rails/rails/blob/v6.0.3.3/actionpack/lib/action_controller/metal/conditional_get.rb#L277

app/controllers/concerns/sends_blob.rb

@@ -44,7 +44,6 @@ module SendsBlob
         Blob::CACHE_TIME
       end
 
-    response.etag = blob.id
     !stale
   end
 

changelogs/unreleased/280553-cache-raw-endpoint.yml

+---
+title: Cache repository raw endpoint
+merge_request: 47225
+author:
+type: changed

spec/controllers/projects/raw_controller_spec.rb

@@ -220,6 +220,23 @@ RSpec.describe Projects::RawController do
         end
       end
     end
+
+    describe 'caching' do
+      def request_file
+        get(:show, params: { namespace_id: project.namespace, project_id: project, id: 'master/README.md' })
+      end
+
+      context 'when If-None-Match header is set' do
+        it 'returns a 304 status' do
+          request_file
+
+          request.headers['If-None-Match'] = response.headers['ETag']
+          request_file
+
+          expect(response).to have_gitlab_http_status(:not_modified)
+        end
+      end
+    end
   end
 
   def execute_raw_requests(requests:, project:, file_path:, **params)