fetch: patch cornercase in children calculation (issue2773)

hgext/fetch.py

@@ -83,7 +83,7 b" def fetch(ui, repo, source='default', **"
         # Is this a simple fast-forward along the current branch?
         newheads = repo.branchheads(branch)
         newchildren = repo.changelog.nodesbetween([parent], newheads)[2]
-        if len(newheads) == 1:
+        if len(newheads) == 1 and len(newchildren):
             if newchildren[0] != parent:
                 return hg.clean(repo, newchildren[0])
             else: