Use correct levels for cap nodes in `put_shard_roots`

The Merkle hashes used for the note commitment trees are domain
separated by level, so when pretending that the subtree roots are leaves
of the cap tree, we need to adjust for their level not being zero.

Closes zcash/librustzcash#874.

Co-authored-by: Sean Bowe <ewillbefull@gmail.com>

zcash_client_sqlite/src/wallet/commitment_tree.rs

@@ -775,9 +775,41 @@ pub(crate) fn put_shard_roots<
     // We treat the cap as a DEPTH-SHARD_HEIGHT tree so that we can make a batch insertion of
     // root data using `Position::from(start_index)` as the starting position and treating the
     // roots as level-0 leaves.
+    #[derive(Clone, Debug, PartialEq, Eq)]
+    struct LevelShifter<H, const SHARD_HEIGHT: u8>(H);
+    impl<H: Hashable, const SHARD_HEIGHT: u8> Hashable for LevelShifter<H, SHARD_HEIGHT> {
+        fn empty_leaf() -> Self {
+            Self(H::empty_root(SHARD_HEIGHT.into()))
+        }
+
+        fn combine(level: Level, a: &Self, b: &Self) -> Self {
+            Self(H::combine(level + SHARD_HEIGHT, &a.0, &b.0))
+        }
+
+        fn empty_root(level: Level) -> Self
+        where
+            Self: Sized,
+        {
+            Self(H::empty_root(level + SHARD_HEIGHT))
+        }
+    }
+    impl<H: HashSer, const SHARD_HEIGHT: u8> HashSer for LevelShifter<H, SHARD_HEIGHT> {
+        fn read<R: io::Read>(reader: R) -> io::Result<Self>
+        where
+            Self: Sized,
+        {
+            H::read(reader).map(Self)
+        }
+
+        fn write<W: io::Write>(&self, writer: W) -> io::Result<()> {
+            self.0.write(writer)
+        }
+    }
+
     let cap = LocatedTree::from_parts(
         Address::from_parts((DEPTH - SHARD_HEIGHT).into(), 0),
-        get_cap(conn, table_prefix).map_err(ShardTreeError::Storage)?,
+        get_cap::<LevelShifter<H, SHARD_HEIGHT>>(conn, table_prefix)
+            .map_err(ShardTreeError::Storage)?,
     );
 
     let cap_result = cap
@@ -785,7 +817,7 @@ pub(crate) fn put_shard_roots<
             Position::from(start_index),
             roots.iter().map(|r| {
                 (
-                    r.root_hash().clone(),
+                    LevelShifter(r.root_hash().clone()),
                     Retention::Checkpoint {
                         id: (),
                         is_marked: false,