localrepo: move extension loading to a separate method

The stateful chg plan [1] requires a special repo object, where ideally all
side effects caused by loading the repo object could be reverted by just
dropping (gabbage collect) the loaded repo object.

Currently, that is impossible because repo.__init__ calls
"extensions.loadall", which may have unpredictable side-effects that cannot
be reverted by dropping the repo object.

This patch moves "extensions.loadall" to a separate method, so chg could
subclass localrepository and make extensions loading a no-op.

[1]: mercurial-scm.org/pipermail/mercurial-devel/2017-February/092547.html

mercurial/localrepo.py

@@ -270,7 +270,7 b' class localrepository(object):'
         self._phasedefaults = []
         try:
             self.ui.readconfig(self.join("hgrc"), self.root)
-            extensions.loadall(self.ui)
+            self._loadextensions()
         except IOError:
             pass
 
@@ -371,6 +371,9 b' class localrepository(object):'
     def close(self):
         self._writecaches()
 
+    def _loadextensions(self):
+        extensions.loadall(self.ui)
+
     def _writecaches(self):
         if self._revbranchcache:
             self._revbranchcache.write()