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

#require emacs
  $ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
  >  -f ert-run-tests-batch-and-exit
  Running 1 tests (*) (glob)
     passed  1/1  hg-test-mode--compilation-mode-support* (glob)
  
  Ran 1 tests, 1 results as expected* (glob)