getBlocks: don't return block if it's not in current tree

[snarfed]

Blocks from old trees that aren't the head commit shouldn't be returned: https://bsky.app/profile/bnewbold.net/post/3kzaknnvjg325 . For example, a record that's been deleted, or (I think) an MST node that's not in the current tree, or a previous commit.