phabricator: use Phabricator's last node information This makes it more strict when checking whether or not we should update a Differential Revision. For example, a) Alice updates D1 to content 1. b) Bob updates D1 to content 2. c) Alice tries to update D1 to content 1. Previously, `c)` will do nothing because `phabsend` detects the patch is not changed. A more correct behavior is to override Bob's update here, hence the patch. This also makes it possible to return a reaonsable "last node" when there is no tags but only `Differential Revision` commit messages. Test Plan: ``` for i in A B C; do echo $i > $i; hg ci -m $i -A $i; done hg phabsend 0:: # D40: created # D41: created # D42: created echo 3 >> C; hg amend; hg phabsend . # D42: updated hg tag --local --hidden -r 2 -f D42 # move tag to the previous version hg phabsend . # D42: skipped (previously it would be "updated") rm -rf .hg; hg init hg phabread --stack D42 | hg import - hg phabsend . # D42: updated hg tag --local --remove D42 hg commit --amend hg phabsend . # D42: updated (no new diff uploaded, previously it will upload a new diff) ``` The old diff object is now returned, which could be useful in the next patch. Differential Revision: https://phab.mercurial-scm.org/D121