sync: only the merge requests that close an issue

The sync took an issue's merge requests from GitLab's related merge requests, which include any MR that merely mentions the issue. The merged sync MR docs!5 (merged) named docs#2 (closed) in its drift report and showed up on that issue as a merged MR. The merge requests now come from GitLab's closed-by list, which holds the MRs that close the issue in any state. Checked against the live API: docs#11 (closed) keeps its merged MR, docs#2 (closed) lists none.

Test first: the new test in tests/test_gitlab.py fails on main and passes here.

Closes #16 (closed)