bundle2: add a ``bundle20.nbparts`` property This property can be used to know how much parts have been added to the bundle2. This will be useful to check if any part have been generated for a push.

bundle2-push: extract changegroup logic in its own function We extract the creation of changegroup related parts into its own function. This precludes the inclusion of more diverse data during the bundle2 push. We use a closure to carry the logic that need to be perform when processing the server reply.

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.

templatekw: add 'subrepos' keyword to show updated subrepositories 'subrepos' template keyword newly added by this patch shows updated subrepositories. For the compatibility with the list of subrepositories shown in the editor at commit: - 'subrepos' is empty, at revisions removing '.hgsub' itself - 'subrepos' is calculated between the revision and the first parent of it, at merge revisions To avoid silent regression, this patch also confirms "hg diff" of ".hgsubstate" and parents for each target revisions in the test.