rebase: only advance phase on successful commit

hgext/rebase.py

@@ -375,6 +375,7 b' def concludenode(repo, rev, p1, p2, comm'
         targetphase = max(ctx.phase(), phases.draft)
         # retractboundary doesn't overwrite upper phase inherited from parent
         newnode = repo[newrev].node()
+        if newnode:
         phases.retractboundary(repo, targetphase, [newnode])
         return newrev
     except util.Abort: