fix: can't access issue detail from closing issue

This MR fixes an issue where the /projects/${project.restId}/merge_requests/${mrId}/closes_issues REST API endpoint returns issue payload that contains fewer field than other issuable responses.

The extension tried to access issue.references.full, but this closes_issues response didn't have the references property.

The solution is to fetch the full issue response.

before	after
closing-issue-before	closing-issue-after

Related to #458 (closed)