config: rename allow_archive to allow-archive
As part of ConfigConsolidationPlan [1], rename the option according to
the new UI guidelines [2] and add an alias for backward compatibility.
[1]: https://www.mercurial-scm.org/wiki/ConfigConsolidationPlan
[2]: https://www.mercurial-scm.org/wiki/UIGuideline#adding_new_options
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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).