relnotes: copy contents of "next" to "5.8" and clear "next"
The same procedure as every year^Wcycle.
Unlike previous cycles, I haven't actually recorded the copy because
we don't want to propagate further changes in "next" (from the stable
branch) to "5.8".
Differential Revision: https://phab.mercurial-scm.org/D10500
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).