context: add a repo accessor

There are 29 instances of 'ctx._repo' in the code, so make the ability
to access more official.

mercurial/context.py

@@ -152,6 +152,8 b' class basectx(object):'
         return hex(self.node())
     def manifest(self):
         return self._manifest
+    def repo(self):
+        return self._repo
     def phasestr(self):
         return phases.phasenames[self.phase()]
     def mutable(self):