exchange: set 'treemanifest' param on pushed changegroups too

In 5c0fd878779c (treemanifests: set bundle2 part parameter indicating
treemanifest, 2016-01-08), I didn't realize I had to set the parameter
separately for getbundle and unbundle. Having the parameter there on
push allows us to push to an empty repo and have the requirements
updated correctly.

mercurial/exchange.py

@@ -717,6 +717,8 b' def _pushb2ctx(pushop, bundler):'
     cgpart = bundler.newpart('changegroup', data=cg)
     if version is not None:
         cgpart.addparam('version', version)
+    if 'treemanifest' in pushop.repo.requirements:
+        cgpart.addparam('treemanifest', '1')
     def handlereply(op):
         """extract addchangegroup returns from server reply"""
         cgreplies = op.records.getreplies(cgpart.id)

tests/test-treemanifest.t

@@ -312,9 +312,28 b' Shelving and unshelving should work'
    1
   +foo
 
+Pushing from treemanifest repo to an empty repo makes that a treemanifest repo
+
+  $ cd ..
+  $ hg init empty-repo
+  $ cat << EOF >> empty-repo/.hg/hgrc
+  > [experimental]
+  > changegroup3=yes
+  > EOF
+  $ grep treemanifest empty-repo/.hg/requires
+  [1]
+  $ hg push -R repo -r 0 empty-repo
+  pushing to empty-repo
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 2 changes to 2 files
+  $ grep treemanifest empty-repo/.hg/requires
+  treemanifest
+
 Create deeper repo with tree manifests.
 
-  $ cd ..
   $ hg --config experimental.treemanifest=True init deeprepo
   $ cd deeprepo