filectx: add a repo accessor

This is similar to a07314472a80 in motivation.  All contexts now have this
method, so the rest of the 'ctx._repo' uses can be converted without worrying
about what type of context it is.

mercurial/context.py

@@ -722,6 +722,8 b' class basefilectx(object):'
         return self._changectx.manifest()
     def changectx(self):
         return self._changectx
+    def repo(self):
+        return self._repo
 
     def path(self):
         return self._path