mergestate.add: store absentfilectxes as nullhex

This is the most natural way to represent these files. We also need to make
sure we don't try to store these files in the merge store.

mercurial/merge.py

@@ -366,6 +366,9 b' class mergestate(object):'
 
         note: also write the local version to the `.hg/merge` directory.
         """
+        if fcl.isabsent():
+            hash = nullhex
+        else:
         hash = util.sha1(fcl.path()).hexdigest()
         self._repo.vfs.write('merge/' + hash, fcl.data())
         self._state[fd] = ['u', hash, fcl.path(),