I have used the tree-level conflict format for several weeks without
problem (after the fix in 51b5d168ae). Now - right after the 0.10.0
release - seems like a good time to enable the config by default.
I enabled the config in our default configs in the CLI crate to reduce
impact on tests (compared to changing the default in `settings.rs`).
This switches the whole `diff_util` module to working with
`MergedTree`, `Merge<Option<TreeValue>>` etc., so it can support
tree-level conflicts.
Since we want to avoid using `ConflictId`s, I switched the hash we use
for conflicts in `--git` style diffs to use an all-'0' id instead of
using the conflict id.
