rebase: change internal format to support destination map 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. Differential Revision: https://phab.mercurial-scm.org/D348