rewriteutil: handle dropped commits when updating description hashes

In looking to leverage this with the absorb extension, the old -> new mapping
there allows the new value to be None.  We could filter that out and not pass it
to this method, but it seems worth a message to the user.  (I wonder if these
should be an info or warning, because it's unlikely people are using `-v`
regularly.)

Differential Revision: https://phab.mercurial-scm.org/D9181

mercurial/rewriteutil.py

@@ -111,8 +111,18 b' def update_hash_refs(repo, commitmsg, pe'
         # We can't make any assumptions about how to update the hash if the
         # cset in question was split or diverged.
         if len(successors) == 1 and len(successors[0]) == 1:
-            newhash = node.hex(successors[0][0])
-            commitmsg = commitmsg.replace(h, newhash[: len(h)])
+            successor = successors[0][0]
+            if successor is not None:
+                newhash = node.hex(successor)
+                commitmsg = commitmsg.replace(h, newhash[: len(h)])
+            else:
+                repo.ui.note(
+                    _(
+                        b'The stale commit message reference to %s could '
+                        b'not be updated\n(The referenced commit was dropped)\n'
+                    )
+                    % h
+                )
         else:
             repo.ui.note(
                 _(