Set `ref_fetched` to `true` in Github::Import#fetch_pull_requests

Since we fetch all the refs we need at the beginning of the import
process, we can safely set this attribute.
Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/github/import.rb

@@ -151,6 +151,7 @@ module Github
               iid: pull_request.iid,
               title: pull_request.title,
               description: description,
+              ref_fetched: true,
               source_project: pull_request.source_project,
               source_branch: pull_request.source_branch_name,
               source_branch_sha: pull_request.source_branch_sha,