Block import/export is broken

[pepyakin]

Using substrate #868889845 I have problems with exporting and importing blocks.

You can reproduce this with syncing flaming fir, like so:

./target/release/substrate
^C
./target/release/substrate export-blocks ff.blocks
./target/release/substrate import-blocks ff.blocks

which then prints an error message

2020-03-31 17:24:04 🔨 Initializing Genesis block/state (state: 0xf0a1…3cf3, header-hash: 0xcc57…d122)
2020-03-31 17:24:04 Loading GRANDPA authority set from genesis on what appears to be first startup.
2020-03-31 17:24:04 Loaded block-time = BabeConfiguration { slot_duration: 3000, epoch_length: 200, c: (1, 4), genesis_authorities: [(Public(00299981a2b92f878baaf5dbeba5c18d4e70f2a1fcd9c61b32ea18daf38f4378 (5C4vDQxA...)), 1), (Public(482a3389a6cf42d8ed83888cfd920fec738ea30f97e44699ada7323f08c3380a (5DhKqkHR...)), 1), (Public(482dbd7297a39fa145c570552249c2ca9dd47e281f0c500c971b59c9dcdcd82e (5DhLtiaQ...)), 1), (Public(6e7e4eb42cbd2e0ab4cae8708ce5509580b8c04d11f6758dbf686d50fe9f9106 (5EZaeQ8d...)), 1)], randomness: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], secondary_slots: true } milliseconds from genesis on first-launch
2020-03-31 17:24:04 Creating empty BABE epoch changes on what appears to be first startup.
2020-03-31 17:24:04 📦 Importing 2480185350008480379 blocks
2020-03-31 17:24:04 Error reading block data at 0: Error decoding field SignedBlock.block