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
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