Don't serialize user displayname as change entry changeuser

[stuartrowe]

Fix for https://issues.jenkins.io/browse/JENKINS-75263.

In order to avoid calling the deprecated and slow hudson.model.User.get(String idOrFullName) attempt to look up a user by id using the p4 login name to match an existing user with hudson.model.User.getById(String id, boolean create). If the the look up by id fails fallback to the existing lookup by fullname.

Testing done

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