Since `Conflict<T>` can also represent a non-conflict state (a single
term), `Merge<T>` seems like better name.
Thanks to @ilyagr for the suggestion in
https://github.com/martinvonz/jj/pull/1774#discussion_r1257547709
Sorry about the churn. It would have been better if I thought of this
name before I introduced `Conflict<T>`.
`MergedTree` is now ready to be used when checking if a commit has
conflicts, and when listing conflicts. We don't yet a way for the user
to say they want to use tree-level conflicts even for these
cases. However, since the backend can decide, we should be able to
have our backend return tree-level conflicts. All writes will still
use path-level conflicts, so the experimentation we can do at Google
is limited.
Beacause `MergedTree` doesn't yet have a way of walking conflicts
while restricting it by a matcher, this will make `jj resolve` a
little slower. I suspect no one will notice.
With `MergedTree`, we can iterate over conflicts by descending into
only the subdirectories that cannot be trivially resolved. We assume
that the trees have previously been resolved as much as possible, so
we don't attempt to resolve conflicts again.
This adds a function for resolving conflicts that can be automatically
resolved, i.e. like our current `merge_trees()` function. However, the
new function is written to merge an arbitrary number of trees and, in
case of unresolvable conflicts, to produce a `Conflict<TreeId>` as
result instead of writing path-level conflicts to the backend. Like
`merge_trees()`, it still leaves conflicts unresolved at the file
level if any hunks conflict, and it resolves paths that can be
trivially resolved even if there are other paths that do conflict.
In order to store conflicts in the commit, as conflicts between a set
of trees, we want to be able merge those trees on the fly. This
introduces a type for that. It has a `Merge(Conflict(Tree))` variant,
where the individual trees cannot have path-level conflicts. It also
has a `Legacy(Tree)` variant, which does allow path-level conflicts. I
think that should help us with the migration.
