memfilectx: override copysource() instead of using dummy nodeid

The "_copied" property in basefilectx is used by renamed() and
copysource(). committablefilectx (which memfilectx subclasses)
overrides renamed() and writes it in terms of copysource() instead of
_copied. That means that the nodeid part of "_copied" is memfilectx is
unused. Let's instead override copysource() too so we don't need the
"_copied".

Differential Revision: https://phab.mercurial-scm.org/D6159

mercurial/context.py

@@ -2393,9 +2393,10 b' class memfilectx(committablefilectx):'
             self._flags = 'x'
         else:
             self._flags = ''
-        self._copied = None
-        if copysource:
-            self._copied = (copysource, nullid)
+        self._copysource = copysource
+
+    def copysource(self):
+        return self._copysource
 
     def cmp(self, fctx):
         return self.data() != fctx.data()