test: use bundle2 for exchange in test-push-warn

Using bundle2 for exchange unlocks the usage of changegroup version 2. This
version of the changegroup held more information (delta base) that result in a
small increase in content size (20 bytes).

tests/test-phases-exchange.t

@@ -1,5 +1,12 b''
 #require killdaemons
 
+  $ cat << EOF >> $HGRCPATH
+  > [experimental]
+  > # drop me once bundle2 is the default,
+  > # added to get test change early.
+  > bundle2-exp = True
+  > EOF
+
   $ hgph() { hg log -G --template "{rev} {phase} {desc} - {node|short}\n" $*; }
 
   $ mkcommit() {
@@ -765,9 +772,9 b' Bare push with next changeset and common'
   searching for changes
   1 changesets found
   uncompressed size of bundle content:
-       172 (changelog)
-       145 (manifests)
-       111  a-H
+       192 (changelog)
+       165 (manifests)
+       131  a-H
   adding changesets
   adding manifests
   adding file changes