It should not be possible to change blockhash before agreement point

[evanlinjin]

The following should not be possible.

bdk_core_staging/bdk_core/tests/test_sparse_chain.rs

Lines 292 to 304 in 35a3347

	#[test]
	fn fix_blockhash_before_agreement_point() {
	let chain1 = chain!([0, h!("im-wrong")], [1, h!("we-agree")]);
	let chain2 = chain!([0, h!("fix")], [1, h!("we-agree")]);
	assert_eq!(
	chain1.determine_changeset(&chain2),
	Ok(changeset! {
	checkpoints: [(0, Some(h!("fix")))],
	txids: []
	})
	)
	}

[LLFourn]

It should not be possible but is it actually desirable to detect and prevent this? It is a "sparse chain" after all. We cannot make guarantees about everything -- for that use a full chain!