discovery: add a devel.discovery.exchange-heads Currently all discovery start with testing local and remote heads. For analysis purpose we make it possible to disable that initial "handshake" and start discovery with the whole repository as undecided. Differential Revision: https://phab.mercurial-scm.org/D9801

test-regression: backing out D9640 (63c923fd7fa8) Since D9640 was pushed, we have seen failure on at least: - test-commit-interactive.t - test-i18n.t - test-log.t The author that change requested it to be removed until he can investigate. For reference: https://phab.mercurial-scm.org/D9640 Backed out changeset 63c923fd7fa8 Differential Revision: https://phab.mercurial-scm.org/D9803