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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2