Apply suggestions from code review

Co-authored-by: str4d <thestr4d@gmail.com>

zcash_client_backend/src/data_api.rs

@@ -1197,7 +1197,7 @@ pub mod testing {
             &self,
             min_confirmations: NonZeroU32,
         ) -> Result<usize, ShardTreeError<Self::Error>> {
-            Ok(usize::try_from(u32::from(min_confirmations) - 1).unwrap())
+            Ok(usize::try_from(u32::from(min_confirmations)).unwrap())
         }
     }
 }

zcash_client_sqlite/src/lib.rs

@@ -805,6 +805,9 @@ impl<P: consensus::Parameters> WalletCommitmentTrees for WalletDb<rusqlite::Conn
     ) -> Result<usize, ShardTreeError<Self::Error>> {
         get_checkpoint_depth(&self.conn, SAPLING_TABLES_PREFIX, min_confirmations)
             .map_err(|e| ShardTreeError::Storage(commitment_tree::Error::Query(e)))?
+            // `CheckpointPruned` is perhaps a little misleading; in this case it's that
+            // the chain tip is unknown, but if that were the case we should never have been
+            // calling this anyway.
             .ok_or(ShardTreeError::Query(QueryError::CheckpointPruned))
     }
 }

zcash_client_sqlite/src/wallet/sapling.rs

@@ -1315,7 +1315,7 @@ pub(crate) mod tests {
         let (account, usk, birthday) = st.test_account().unwrap();
         let dfvk = st.test_account_sapling().unwrap();
 
-        // Generate a block that with funds belonging to our wallet.
+        // Generate a block with funds belonging to our wallet.
         st.generate_next_block(
             &dfvk,
             AddressType::DefaultExternal,