config: add defaultvalue template keyword This patch tries to fix one of the issues mentioned in issue6014. This adds a new `defaultvalue` template keyword to be used with `hg showconfig` to get the default value of the config item. Differential Revision: https://phab.mercurial-scm.org/D6704

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    "arc.land.onto.default": "@",
    "base": "hg:.^"
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