overlayworkingctx: move _wrappedctx out of the constructor

With rebase, we will be setting the _wrappedctx at a different point from the
wctx construction (somewhat later, and possibly several times). Move it to a
public function.

Differential Revision: https://phab.mercurial-scm.org/D1231

mercurial/context.py

@@ -1990,11 +1990,14 b' class overlayworkingctx(workingctx):'
     is `False`, the file was deleted.
     """
 
-    def __init__(self, repo, wrappedctx):
+    def __init__(self, repo):
         super(overlayworkingctx, self).__init__(repo)
         self._repo = repo
         self.clean()
+
+    def setbase(self, wrappedctx):
         self._wrappedctx = wrappedctx
+        self._parents = [wrappedctx]
 
     def data(self, path):
         if self.isdirty(path):