clonebundle: support bundle2

exchange.readbundle() can return 2 different types. We weren't handling
the bundle2 case. Handle it.

At some point we'll likely want a generic API for applying a bundle from
a file handle. For now, create another one-off until we figure out what
the unified bundle API should look like (addressing this is a can of
worms I don't want to open right now).

mercurial/exchange.py

@@ -1653,6 +1653,10 b' def trypullbundlefromurl(ui, repo, url):'
             try:
                 fh = urlmod.open(ui, url)
                 cg = readbundle(ui, fh, 'stream')
+
+                if isinstance(cg, bundle2.unbundle20):
+                    bundle2.processbundle(repo, cg, lambda: tr)
+                else:
                 changegroup.addchangegroup(repo, cg, 'clonebundles', url)
                 tr.close()
                 return True

tests/test-clonebundles.t

@@ -128,7 +128,7 b' Bundle with partial content works'
 
 Bundle with full content works
 
-  $ hg -R server bundle --type gzip --base null -r tip full.hg
+  $ hg -R server bundle --type gzip-v2 --base null -r tip full.hg
   2 changesets found
 
   $ echo "http://localhost:$HGPORT1/full.hg" > server/.hg/clonebundles.manifest