phases: use revision number in `_pushdiscoveryphase` We now reach our target checkpoint in terms of rev-num conversion. The `_pushdiscoveryphase` function is now performing graph computation based on revision number only. Avoiding repeated conversion from node-id to rev-num. See previous changeset updated `new_heads` for rationnal. Again, time saved in the 100 milliseconds order of magnitude for the mozilla-try benchmark I have been using. However, wow that the logic is done using revision number, we can look into having better logic in the next changesets, which will provide a much bigger speedup.

https://bz.mercurial-scm.org/1089

  $ hg init repo
  $ cd repo
  $ mkdir a
  $ echo a > a/b
  $ hg ci -Am m
  adding a/b

  $ hg rm a
  removing a/b
  $ hg ci -m m a

  $ mkdir a b
  $ echo a > a/b
  $ hg ci -Am m
  adding a/b

  $ hg rm a
  removing a/b
  $ cd b

Relative delete:

  $ hg ci -m m ../a

  $ cd ..