bundle2: call _pushbundle2extraparts a bit sooner

This is the first step of a refactoring that will ease the inclusion of new part
in the bundle2 push and includes more information (like phases) in this push

We need to move the function a bit sooner to be able to group the generation of
`b2x:check:heads` and `b2x:changegroup` part in an external function. We move it
sooner to preserve parts creation order bundle2 tests rely on. At the ends of this
refactoring the `_pushbundle2extraparts` will be replaced by another mechanism
anyway.

mercurial/exchange.py

@@ -212,10 +212,10 b' def _pushbundle2(pushop):'
     # create reply capability
     capsblob = bundle2.encodecaps(pushop.repo.bundle2caps)
     bundler.newpart('b2x:replycaps', data=capsblob)
+    extrainfo = _pushbundle2extraparts(pushop, bundler)
     # Send known heads to the server for race detection.
     if not pushop.force:
         bundler.newpart('B2X:CHECK:HEADS', data=iter(pushop.remoteheads))
-    extrainfo = _pushbundle2extraparts(pushop, bundler)
     # add the changegroup bundle
     cg = changegroup.getlocalbundle(pushop.repo, 'push', pushop.outgoing)
     cgpart = bundler.newpart('B2X:CHANGEGROUP', data=cg.getchunks())