After the previous commit, `MergedTree` and `MergedTreeId` are almost
identical, with the only difference being that `MergedTree` is attached
to a `Store` instance. `MergedTreeId` is also equivalent to
`Merge<TreeId>`, since it is just a wrapper around it.
In the future, `MergedTree` might contain additional metadata like
conflict labels. Therefore, I replaced `MergedTreeId` with `MergedTree`
wherever I think it would be required to pass this additional metadata,
or where the additional methods provided by `MergedTree` would be
useful. In any remaining places, I replaced it with `Merge<TreeId>`.
I also renamed some of the `tree_id()` methods to `tree_ids()` for
consistency, since now they return a merge of individual tree IDs
instead of a single "merged tree ID". Similarly, `MergedTree` no longer
has an `id()` method, since tree IDs won't fully identify a `MergedTree`
once it contains additional metadata.
