evolve: don't embed graft data when completing interrupted evolve operations This data shows in TortoiseHg, and is a bit awkward since it wasn't added before in the non interrupted case. It originated before 823031b51d81, when the implementation changed away from using the graft command. According to @marmoute, unconditionally adding `source` served as noise to create a new hash and prevent a cycle, so use the same key (`rebase_source`) as would happen in the uninterrupted case (as demonstrated by the first change in test-stabilize-conflict.t).