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
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev linkrev nodeid p1 p2
0 0 b80de5d13875 000000000000 000000000000
1 1 0376abec49b8 000000000000 000000000000
$ cd ..