bundle: avoid crash when no good changegroup version found

When using treemanifests, only changegroup3 bundles can be
created. However, there is currently no way of requesting a
changegroup3 bundle, so we run into an assertion in
changegroup.getbundler() when trying to get a changroup2
bundler. Let's avoid the traceback and print a short error message
instead.

mercurial/commands.py

@@ -1399,6 +1399,10 b' def bundle(ui, repo, fname, dest=None, *'
         base = scmutil.revrange(repo, opts.get('base'))
     # TODO: get desired bundlecaps from command line.
     bundlecaps = None
+    if cgversion not in changegroup.supportedoutgoingversions(repo):
+        raise error.Abort(_("repository does not support bundle version %s") %
+                          cgversion)
+
     if base:
         if dest:
             raise error.Abort(_("--base is incompatible with specifying "

tests/test-treemanifest.t

@@ -736,3 +736,9 b' Packed bundle'
   bundle requirements: generaldelta, revlogv1, treemanifest
   $ hg debugbundle --spec repo-packed.hg
   none-packed1;requirements%3Dgeneraldelta%2Crevlogv1%2Ctreemanifest
+
+Bundle with changegroup2 is not supported
+
+  $ hg -R deeprepo bundle --all -t v2 deeprepo.bundle
+  abort: repository does not support bundle version 02
+  [255]