What default shell to use for an SSH user limited to a single command

When limiting a user to a single command via ssh, what default shell should be used for the user? This is a follow up to how can shellshock be exploited over SSH? I have a machine that is running an ssh server and has a dummy user. I have limited this dummy user to running a single command over ssh by specifying a command option in the authorized_keys file. I have been using bash as the default shell of the dummy user, but since the shellshock bug, I have been thinking maybe there is a more secure shell. The dummy user is only allowed to run an rsync command so I would image it would run on most shells.