relnotes: add release notes for relevant changes I did since the 5.4 release After having written the notes, I realized that the added points contain changes that were part of the 5.4.2 release. These were never included in any release notes. I asked on IRC whether they should be added to this file or not, but didn’t get an answer to this question. If they should not be added here, I can remove them. Differential Revision: https://phab.mercurial-scm.org/D8770