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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
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