This is a pure refactor, motivated by wanting to introduce to the
BufferStore new logic for opening staged and committed changes.
I found the `BufferStoreImpl` trait a little bit confusing, particularly
how the different implementors of the trait held a handle back to the
owning buffer store. I was able to reduce the amount of code
considerably (-78 lines) by using a two-variant enum instead, similar to
what we do for `LspStore`, `WorktreeStore` and `Worktree`.
Release Notes:
- N/A
91a565f5fa