D6704: config: add defaultvalue template keyword

[Yuya Nishihara]

>   `hg config -T '{defaultvalue}'` shows a bunch of entries like `<object object at 0x7fc4295c00f0>`, which is clearly not something we want to show the user. Also `hg config -T json` crashes for a similar reason (`mercurial.error.ProgrammingError: cannot encode <object object at 0x7fdd1a6a20f0>`).

Yup. That's mostly because of configitems.dynamicdefault.