exchange: turn on option that makes concurrent pushes work better The motivation is simply to make hg work better out of the box. This is a slight backwards compatibility break, because client extensions could have assumed that the list of heads the client sees during discovery will be the list of heads during the entirety of the push. It seems unlikely to matter, and not worth mentioning. There's a fair amount of diff in tests, but this is just due to sending a few more bytes on the wire, except for test-acl.t. The extra "invalid branch cache" lines in test-acl.t don't seem to indicate a problem: the branchcache now get computed during the bundle application (because of the check:updated-heads bundle part), but doesn't get rolled back when transactions rollback, thus causing a message in the next operation computing the branch cache. Before this change, I assume the branchcache was only computed on transaction commit, so not computed at all when the transactions roll back, thus no messages. Differential Revision: https://phab.mercurial-scm.org/D8202