config: have a way to backup and restore value in config

This is introduce to allow temporary overwriting of a config value while being
able to reinstall the old value once done. The main advantage over using
``config`` and ``setconfig`` is that backup and restore will properly restore
the lack of any config. Restoring the fact that there was no value is important
to allow config user to keep using meaniful default value.

A more naive approach will result in the following scenario::

  Before:
    config(section, item, my_default) --> my_default

  temporal overwrite
    old = config(section, item)
    …
    setconfig(section, item, old)

  After
    config(section, item, my_default) --> None

The first user of this feature should be mq to overwriting minimal phase of
future commit.

mercurial/config.py

@@ -61,6 +61,19 b' class config(object):'
         self._source.update(src._source)
     def get(self, section, item, default=None):
         return self._data.get(section, {}).get(item, default)
+
+    def backup(self, section, item):
+        """return a tuple allowing restore to reinstall a previous valuesi
+
+        The main reason we need it is because it handle the "no data" case.
+        """
+        try:
+            value = self._data[section][item]
+            source = self.source(section, item)
+            return (section, item, value, source)
+        except KeyError:
+            return (section, item)
+
     def source(self, section, item):
         return self._source.get((section, item), "")
     def sections(self):
@@ -73,6 +86,20 b' class config(object):'
         self._data[section][item] = value
         self._source[(section, item)] = source
 
+    def restore(self, data):
+        """restore data returned by self.backup"""
+        if len(data) == 4:
+            # restore old data
+            section, item, value, source = data
+            self._data[section][item] = value
+            self._source[(section, item)] = source
+        else:
+            # no data before, remove everything
+            section, item = data
+            if section in self._data:
+                del self._data[section][item]
+            self._source.pop((section, item), None)
+
     def parse(self, src, data, sections=None, remap=None, include=None):
         sectionre = re.compile(r'\[([^\[]+)\]')
         itemre = re.compile(r'([^=\s][^=]*?)\s*=\s*(.*\S|)')

mercurial/ui.py

@@ -142,6 +142,15 b' class ui(object):'
             self._trustusers.update(self.configlist('trusted', 'users'))
             self._trustgroups.update(self.configlist('trusted', 'groups'))
 
+    def backupconfig(self, section, item):
+        return (self._ocfg.backup(section, item),
+                self._tcfg.backup(section, item),
+                self._ucfg.backup(section, item),)
+    def restoreconfig(self, data):
+        self._ocfg.restore(data[0])
+        self._tcfg.restore(data[1])
+        self._ucfg.restore(data[2])
+
     def setconfig(self, section, name, value, overlay=True):
         if overlay:
             self._ocfg.set(section, name, value)