rewriteutil: also consider pending obsoletes when updating hashes in messages

Phabricator builds up the replacement commits and mapping in a single
transaction, and then finalizes everything once the commits have been rewritten.
That's too late when trying to update the messages for those commits.

I'm a little concerned that this isn't a generic enough interface, since it
doesn't mimic the list of list return of `obsutil.successorssets()`.  But this
is the type of mapping that phabricator maintains, and I don't think the methods
that would be interested in calling this need to worry about split and
divergence.  We can fix that later if the need arises.

Differential Revision: https://phab.mercurial-scm.org/D8949

mercurial/rewriteutil.py

@@ -79,12 +79,18 b' def skip_empty_successor(ui, command):'
         )
 
 
-def update_hash_refs(repo, commitmsg):
+def update_hash_refs(repo, commitmsg, pending=None):
     """Replace all obsolete commit hashes in the message with the current hash.
 
     If the obsolete commit was split or is divergent, the hash is not replaced
     as there's no way to know which successor to choose.
+
+    For commands that update a series of commits in the current transaction, the
+    new obsolete markers can be considered by setting ``pending`` to a mapping
+    of ``pending[oldnode] = [successor_node1, successor_node2,..]``.
     """
+    if not pending:
+        pending = {}
     cache = {}
     sha1s = re.findall(sha1re, commitmsg)
     unfi = repo.unfiltered()
@@ -94,8 +100,12 b' def update_hash_refs(repo, commitmsg):'
             continue
         ctx = unfi[fullnode]
         if not ctx.obsolete():
+            successors = pending.get(fullnode)
+            if successors is None:
             continue
-
+            # obsutil.successorssets() returns a list of list of nodes
+            successors = [successors]
+        else:
         successors = obsutil.successorssets(repo, ctx.node(), cache=cache)
 
         # We can't make any assumptions about how to update the hash if the