ui: add a "config_default" method
This allow code to access the expected value for a config. This is useful in the
context of dynamic default value, and short term it will be useful to write
dynamically "correct" code.
Differential Revision: https://phab.mercurial-scm.org/D9759
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors