config: add experimental argument to the config registrar Until now, there are almost 28 config items which are considered as `experimental` but, not present in the `experimental` section of the registrar. This patch adds an `experimental` argument to the config registrar to mark such config items. Differential Revision: https://phab.mercurial-scm.org/D6728 Differential Revision: https://phab.mercurial-scm.org/D6746