bundle2: handle empty 'b2x:changegroup' value in push and pull

Changeset e4dc2b0be056 added advertising of supported changegroup version
through the new 'b2x:changegroup' capability. However, this capability is not
new and has been around since 3.1 with an empty value. This makes new clients
unable to push to 3.2 servers through bundle2 as they cannot find a common
changegroup version to use from and empty list.

Treating empty 'b2x:changegroup' value as old client fixes it.

mercurial/exchange.py

@@ -448,7 +448,7 b' def _pushb2ctx(pushop, bundler):'
     b2caps = bundle2.bundle2caps(pushop.remote)
     version = None
     cgversions = b2caps.get('b2x:changegroup')
-    if cgversions is None:
+    if not cgversions:  # 3.1 and 3.2 ship with an empty value
         cg = changegroup.getlocalchangegroupraw(pushop.repo, 'push',
                                                 pushop.outgoing)
     else:
@@ -1200,7 +1200,7 b' def _getbundlechangegrouppart(bundler, r'
         # build changegroup bundle here.
         version = None
         cgversions = b2caps.get('b2x:changegroup')
-        if cgversions is None:
+        if not cgversions:  # 3.1 and 3.2 ship with an empty value
             cg = changegroup.getchangegroupraw(repo, source, heads=heads,
                                                common=common,
                                                bundlecaps=bundlecaps)