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.