rebase: update the model when using "git cola rebase"

Get the completion widget working by ensuring that we've updated our
branches and tags before running the Rebase command.

The Rebase command uses GitRefDialog, which needs the model to have been
refreshed.

cola/main.py

@@ -693,6 +693,7 @@ def cmd_rebase(args):
         'branch': args.branch,
     }
     context = app.application_init(args)
+    context.model.update_refs()
     status, _, _ = cmds.do(cmds.Rebase, context, **kwargs)
     return status