The added test cases would fail before this fix:

    --- FAIL: TestDecodeError/Empty (0.00s)
        unmarshal_test.go:28: Decode("") got "EOF", want "unexpected EOF"
    --- FAIL: TestDecodeError/NoSpace (0.00s)
        unmarshal_test.go:28: Decode("foo") got "EOF", want "unexpected EOF"

They would still return an error, but they'd just return io.EOF.
While that error signals that we've hit the end of input,
it doesn't signal to the codec user that something went wrong.
An io.EOF is not a worrying error, in many situations.

If the input is empty, or is too short to be a valid git object,
we should return a clear error to signal the operation did not work.
io.ErrUnexpectedEOF is unambiguous,
and will make go-ipfs start erroring on this edge case.

For ipfs/kubo#8378.