squash: decide to abandon source commit if entire diff was selected

Before this patch, we would abandon the source commit if it became
empty after applying the reverse diff. This changes that condition to
the equivalent condition of the selected tree being the source
commit's original tree. This will help us rewrite the code to use
`transform_descendants()`.

cli/src/commands/squash.rs

@@ -203,15 +203,15 @@ from the source will be moved into the destination.
             tx.format_commit_summary(source),
             tx.format_commit_summary(destination)
         );
-        let new_parent_tree_id =
+        let selected_tree_id =
             diff_selector.select(&parent_tree, &source_tree, matcher, Some(&instructions))?;
-        let new_parent_tree = tx.repo().store().get_root_tree(&new_parent_tree_id)?;
+        let selected_tree = tx.repo().store().get_root_tree(&selected_tree_id)?;
+        let abandon_source = selected_tree.id() == source_tree.id();
         // TODO: Do we want to optimize the case of moving to the parent commit (`jj
         // squash -r`)? The source tree will be unchanged in that case.
 
         // Apply the reverse of the selected changes onto the source
-        let new_source_tree = source_tree.merge(&new_parent_tree, &parent_tree)?;
-        let abandon_source = new_source_tree.id() == parent_tree.id();
+        let new_source_tree = source_tree.merge(&selected_tree, &parent_tree)?;
         if abandon_source {
             abandoned_commits.push(source);
             tx.mut_repo().record_abandoned_commit(source.id().clone());
@@ -221,8 +221,8 @@ from the source will be moved into the destination.
                 .set_tree_id(new_source_tree.id().clone())
                 .write()?;
         }
-        if new_parent_tree_id != parent_tree.id() {
-            tree_diffs.push((parent_tree, new_parent_tree));
+        if selected_tree_id != parent_tree.id() {
+            tree_diffs.push((parent_tree, selected_tree));
         }
     }
     if tree_diffs.is_empty() {