discovery: indices between sample and yesno must match (issue4438)

3ef893520a85 changed 'sample' from a list to a set. The iteration order is thus
undefined and the yesno indices are not stable.

To solve this, repeat the listification and comment from elsewhere in the code.

Note: the randomness in the discovery protocol can make this problem hard to
reproduce.

mercurial/setdiscovery.py

@@ -134,6 +134,8 b' def findcommonheads(ui, local, remote,'
     roundtrips += 1
     ownheads = dag.heads()
     sample = _limitsample(ownheads, initialsamplesize)
+    # indices between sample and externalized version must match
+    sample = list(sample)
     if remote.local():
         # stopgap until we have a proper localpeer that supports batch()
         srvheadhashes = remote.heads()