tests/test-changelog-exec.t

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

#require execbit

b51a8138292a introduced a regression where we would mention in the
changelog executable files added by the second parent of a merge. Test
that that doesn't happen anymore

  $ hg init repo
  $ cd repo
  $ echo foo > foo
  $ hg ci -qAm 'add foo'

  $ echo bar > bar
  $ chmod +x bar
  $ hg ci -qAm 'add bar'

manifest of p2:

  $ hg manifest
  bar
  foo

  $ hg up -qC 0
  $ echo >> foo
  $ hg ci -m 'change foo'
  created new head

manifest of p1:

  $ hg manifest
  foo

  $ hg merge
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ chmod +x foo
  $ hg ci -m 'merge'

this should not mention bar but should mention foo:

  $ hg tip -v
  changeset:   3:c53d17ff3380
  tag:         tip
  parent:      2:ed1b79f46b9a
  parent:      1:d394a8db219b
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  files:       foo
  description:
  merge
  
  

  $ hg debugindex bar
     rev    offset  length  ..... linkrev nodeid       p1           p2 (re)
       0         0       5  .....       1 b004912a8510 000000000000 000000000000 (re)

  $ cd ..