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

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# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL