pr: Do not exceed GitHub comment mention limit

Fixes: https://trac.macports.org/ticket/63821

Author: chrstphrchvz

pr/webhook/pull_request.go

@@ -117,17 +117,26 @@ func (receiver *Receiver) processPullRequest(event *github.PullRequestEvent) {
 			mentionSymbol = "@"
 		}
 		if len(handles) > 0 && !strings.Contains(*event.PullRequest.Body, "[skip notification]") {
-			body := "Notifying maintainers:\n"
+			// GitHub limits mentions to 50 per comment
+			const MentionLimit := 50
+			i := 0
+			var body string
 			for handle, ports := range handles {
+				if i % MentionLimit == 0 {
+					body = "Notifying maintainers:\n"
+				}
 				body += mentionSymbol + handle + " for port " + strings.Join(ports, ", ") + ".\n"
 				err = receiver.githubClient.AddAssignees(owner, repo, number, []string{handle})
 				if err != nil {
 					log.Println(err)
 				}
-			}
-			err = receiver.githubClient.CreateComment(owner, repo, number, &body)
-			if err != nil {
-				log.Println(err)
+				i++
+				if i % MentionLimit == 0 || i == len(handles) {
+					err = receiver.githubClient.CreateComment(owner, repo, number, &body)
+					if err != nil {
+						log.Println(err)
+					}
+				}
 			}
 		}