ui: add a "config_default" method

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

--- a/mercurial/ui.py	Thu Jan 14 11:46:30 2021 +0100
+++ b/mercurial/ui.py	Wed Jan 13 18:29:19 2021 +0100
@@ -673,6 +673,18 @@
                     )
         return value
 
+    def config_default(self, section, name):
+        """return the default value for a config option
+
+        The default is returned "raw", for example if it is a callable, the
+        callable was not called.
+        """
+        item = self._knownconfig.get(section, {}).get(name)
+
+        if item is None:
+            raise KeyError((section, name))
+        return item.default
+
     def configsuboptions(self, section, name, default=_unset, untrusted=False):
         """Get a config option and all sub-options.