config: add hasconfig method and supporting plumbing

We add the hasconfig method to make it possible to distinguish between a
config value that was never supplied and one that is empty.

mercurial/config.py

@@ -30,6 +30,8 b' class config(object):'
         return config(self)
     def __contains__(self, section):
         return section in self._data
+    def hasitem(self, section, item):
+        return item in self._data.get(section, {})
     def __getitem__(self, section):
         return self._data.get(section, {})
     def __iter__(self):

mercurial/ui.py

@@ -507,6 +507,9 b' class ui(object):'
                 result = default or []
         return result
 
+    def hasconfig(self, section, name, untrusted=False):
+        return self._data(untrusted).hasitem(section, name)
+
     def has_section(self, section, untrusted=False):
         '''tell whether section exists in config.'''
         return section in self._data(untrusted)