push: restrict common discovery to the pushed set This changeset make use of the ability of the set discovery to only search common changeset for a subset of the repository. Restricting that search to the pushed set avoid potential waste of time finding out the status of many unrelated related revision. Repository with many heads were especially badly affected by this. Here is an example of findcommonhead discovery for pushing 11 outgoing changeset on a repository with tens of thousand of unrelated heads. (discovery run over a ssh link to localhost). Before: queries: 92 time: 44.1996s After: queries: 3 time: 0.6938s A x63 speedup even with a network link without latency.