feat(chain): Add initial_changeset to graphs

Add `initial_changeset` to TxGraph and IndexedTxGraph

crates/chain/tests/test_indexed_tx_graph.rs

@@ -65,16 +65,20 @@ fn insert_relevant_txs() {
 
     let txs = [tx_c, tx_b, tx_a];
 
+    let changeset = indexed_tx_graph::ChangeSet {
+        graph: tx_graph::ChangeSet {
+            txs: txs.clone().into(),
+            ..Default::default()
+        },
+        indexer: keychain::ChangeSet([((), 9_u32)].into()),
+    };
+
     assert_eq!(
         graph.insert_relevant_txs(txs.iter().map(|tx| (tx, None)), None),
-        indexed_tx_graph::ChangeSet {
-            graph: tx_graph::ChangeSet {
-                txs: txs.into(),
-                ..Default::default()
-            },
-            indexer: keychain::ChangeSet([((), 9_u32)].into()),
-        }
-    )
+        changeset,
+    );
+
+    assert_eq!(graph.initial_changeset(), changeset,);
 }
 
 #[test]

crates/chain/tests/test_keychain_txout_index.rs

@@ -43,6 +43,8 @@ fn spk_at_index(descriptor: &Descriptor<DescriptorPublicKey>, index: u32) -> Scr
 
 #[test]
 fn test_set_all_derivation_indices() {
+    use bdk_chain::indexed_tx_graph::Indexer;
+
     let (mut txout_index, _, _) = init_txout_index();
     let derive_to: BTreeMap<_, _> =
         [(TestKeychain::External, 12), (TestKeychain::Internal, 24)].into();
@@ -56,6 +58,7 @@ fn test_set_all_derivation_indices() {
         keychain::ChangeSet::default(),
         "no changes if we set to the same thing"
     );
+    assert_eq!(txout_index.initial_changeset().as_inner(), &derive_to);
 }
 
 #[test]

crates/chain/tests/test_tx_graph.rs

@@ -141,7 +141,7 @@ fn insert_txouts() {
         changeset,
         ChangeSet {
             txs: [update_txs.clone()].into(),
-            txouts: update_ops.into(),
+            txouts: update_ops.clone().into(),
             anchors: [(conf_anchor, update_txs.txid()), (unconf_anchor, h!("tx2"))].into(),
             last_seen: [(h!("tx2"), 1000000)].into()
         }
@@ -186,6 +186,17 @@ fn insert_txouts() {
         )]
         .into()
     );
+
+    // Check that the initial_changeset is correct
+    assert_eq!(
+        graph.initial_changeset(),
+        ChangeSet {
+            txs: [update_txs.clone()].into(),
+            txouts: update_ops.into_iter().chain(original_ops).collect(),
+            anchors: [(conf_anchor, update_txs.txid()), (unconf_anchor, h!("tx2"))].into(),
+            last_seen: [(h!("tx2"), 1000000)].into()
+        }
+    );
 }
 
 #[test]