Implement GetLinkedIssuesAsync in GitLab provider

Author: Jericho

src/GitReleaseManager.Core/Provider/GitLabProvider.cs

@@ -391,10 +391,23 @@ public string GetIssueType(Issue issue)
             return issue.IsPullRequest ? "Merge Request" : "Issue";
         }
 
-        public async Task<Issue> GetLinkedIssueAsync(string owner, string repository, int issueNumber)
+        public Task<IEnumerable<Issue>> GetLinkedIssuesAsync(string owner, string repository, Issue issue)
         {
-            // This is a placeholder until I figure out if GitLab has a something equivalent to GitHub's "linked issue"
-            throw new NotImplementedException();
+            return ExecuteAsync(() =>
+            {
+                if (issue.IsPullRequest)
+                {
+                    var closes = _gitLabClient.MergeRequests.ClosesIssues(issue.PublicNumber);
+                    var issues = _mapper.Map<IEnumerable<Issue>>(closes);
+                    return Task.FromResult(issues);
+                }
+                else
+                {
+                    var closedBy = _gitLabClient.Issues.ClosedBy(GetGitLabProjectId(owner, repository), issue.PublicNumber);
+                    var issues = _mapper.Map<IEnumerable<Issue>>(closedBy);
+                    return Task.FromResult(issues);
+                }
+            });
         }
 
         private int GetGitLabProjectId(string owner, string repository)