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
% lazy ancestor set for [], stoprev = 0, inclusive = False
membership: []
iteration: []
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = False
membership: [7, 8, 3, 4, 1, 0]
iteration: [3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [1, 3], stoprev = 0, inclusive = False
membership: [1, 0]
iteration: [0, 1]
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = True
membership: [11, 13, 7, 8, 3, 4, 1, 0]
iteration: [11, 13, 3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = False
membership: [7, 8]
iteration: [7, 8]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = True
membership: [11, 13, 7, 8]
iteration: [11, 13, 7, 8]