subrepo: detect issue3781 case earlier so it apply to bundle2

We are doing some strange special casing of phase push when:

- the source is a subrepo
- the destination is publishing
- some changeset are still draft on the destination

In that case we do not push phases information (to publish the draft changesets)
because it could break simple cycle of 'clone/pull/push' of subrepos. We have to
detect this case earlier to have bundle2 respecting it.

We change the test to check the behavior for both bundle1 and bundle2.

mercurial/exchange.py

@@ -305,6 +305,20 b' def _pushdiscoveryphase(pushop):'
     unfi = pushop.repo.unfiltered()
     remotephases = pushop.remote.listkeys('phases')
     publishing = remotephases.get('publishing', False)
+    if (pushop.ui.configbool('ui', '_usedassubrepo', False)
+        and remotephases    # server supports phases
+        and not pushop.outgoing.missing # no changesets to be pushed
+        and publishing):
+        # When:
+        # - this is a subrepo push
+        # - and remote support phase
+        # - and no changeset are to be pushed
+        # - and remote is publishing
+        # We may be in issue 3871 case!
+        # We drop the possible phase synchronisation done by
+        # courtesy to publish changesets possibly locally draft
+        # on the remote.
+        remotephases = {'publishing': 'True'}
     ana = phases.analyzeremotephases(pushop.repo,
                                      pushop.fallbackheads,
                                      remotephases)

tests/test-subrepo.t

@@ -1490,7 +1490,17 b' Courtesy phases synchronisation to publi'
   > [paths]
   > default=../issue3781-dest/
   > EOF
-  $ hg push
+  $ hg push --config experimental.bundle2-exp=False
+  pushing to $TESTTMP/issue3781-dest (glob)
+  pushing subrepo s to $TESTTMP/issue3781-dest/s
+  searching for changes
+  no changes found
+  searching for changes
+  no changes found
+  [1]
+# clean the push cache
+  $ rm s/.hg/cache/storehash/*
+  $ hg push --config experimental.bundle2-exp=True
   pushing to $TESTTMP/issue3781-dest (glob)
   pushing subrepo s to $TESTTMP/issue3781-dest/s
   searching for changes