RFR: 1611: Improve adding and removing of labels in GitLab

[Zhao Song]

On Wed, 12 Oct 2022 20:59:07 GMT, Zhao Song <zsong at openjdk.org> wrote:

> Improved adding and removing of labels in GitLabMergeRequest.
> 
> And make the label handling consistent with GitHubPullRequest.

forge/src/main/java/org/openjdk/skara/forge/gitlab/GitLabMergeRequest.java line 706:

> 704:                     .sorted()
> 705:                     .collect(Collectors.toList());
> 706:         }

I don't know if we need to make it consistent with GitHubPullRequest but I did.

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PR: https://git.openjdk.org/skara/pull/1396