A later patch will add multiple destination support. This patch changes
internal state and the rebase state file format to support that. But the
external interface still only supports single destination.
A test was added to make sure rebase still supports legacy state file.
The new state file is incompatible with old clients. We had done similar
state file format change before: 5eac7ab, 92409f8, and 72412af. The state
file is transient, so the impact of incompatibility is limited. Besides,
the old client won't support multiple destinations anyway so it does not
really make sense to make the file format compatible with them.