tests/test-rebuildstate.t

setdiscovery: return anyincoming=False when remote's only head is nullid

This fixes (issue2907) a crash when using 'hg incoming --bundle' with an empty
remote repo and a non-empty local repo.

This also fixes an unreported bug that 'hg summary --remote' erroneously
reports incoming changes when the remote repo is empty and the local is not.

Also, add a test to make sure issue2907 stays fixed

basic test for hg debugrebuildstate

  $ hg init repo
  $ cd repo

  $ touch foo bar
  $ hg ci -Am 'add foo bar'
  adding bar
  adding foo

  $ touch baz
  $ hg add baz
  $ hg rm bar

  $ hg debugrebuildstate

state dump after

  $ hg debugstate --nodates | sort
  n 666         -1 bar
  n 666         -1 foo

status

  $ hg st -A
  ! bar
  ? baz
  C foo