context: add missing manifest invalidation after write in overlayworkingctx

This was breaking my merge-diff logic that will be in the next patch.

Differential Revision: https://phab.mercurial-scm.org/D9908

mercurial/context.py

@@ -2597,6 +2597,7 b' class overlayworkingctx(committablectx):'
             b'flags': flags,
             b'copied': copied,
         }
+        util.clearcachedproperty(self, b'_manifest')
 
     def filectx(self, path, filelog=None):
         return overlayworkingfilectx(