bundle2caps: advertise the available versions for changegroup packer

This will let the bundle2 client and server detect what packer they should be using.

This detection part is not done. I expect it to be done with the addition of the
second packer (with generaldelta support).

mercurial/bundle2.py

@@ -882,6 +882,7 b' def getrepocaps(repo):'
     Exists to allow extensions (like evolution) to mutate the capabilities.
     """
     caps = capabilities.copy()
+    caps['b2x:changegroup'] = tuple(sorted(changegroup.packermap.keys()))
     if obsolete.isenabled(repo, obsolete.exchangeopt):
         supportedformat = tuple('V%i' % v for v in obsolete.formats)
         caps['b2x:obsmarkers'] = supportedformat