rewrite: optimize for rebase onto metadata-only change

When you run e.g. `jj describe <some old commit>` or `jj squash -r
<some old commit>`, the descendants' tree objects will not change, so
we can avoid calculating them. This speeds up rebasing of 126 commits
in the git.git repo from ~9.8 s to ~3.6 s.

lib/src/commit.rs

@@ -18,7 +18,7 @@ use std::hash::{Hash, Hasher};
 use std::sync::Arc;
 
 use crate::backend;
-use crate::backend::{ChangeId, CommitId, Signature};
+use crate::backend::{ChangeId, CommitId, Signature, TreeId};
 use crate::repo_path::RepoPath;
 use crate::store::Store;
 use crate::tree::Tree;
@@ -112,6 +112,10 @@ impl Commit {
             .unwrap()
     }
 
+    pub fn tree_id(&self) -> &TreeId {
+        &self.data.root_tree
+    }
+
     pub fn change_id(&self) -> &ChangeId {
         &self.data.change_id
     }

lib/src/rewrite.rs

@@ -61,10 +61,24 @@ pub fn rebase_commit(
     new_parents: &[Commit],
 ) -> Commit {
     let store = mut_repo.store();
-    let old_base_tree = merge_commit_trees(mut_repo.as_repo_ref(), &old_commit.parents());
-    let new_base_tree = merge_commit_trees(mut_repo.as_repo_ref(), new_parents);
-    // TODO: pass in labels for the merge parts
-    let new_tree_id = merge_trees(&new_base_tree, &old_base_tree, &old_commit.tree()).unwrap();
+    let old_parents = old_commit.parents();
+    let old_parent_trees = old_parents
+        .iter()
+        .map(|parent| parent.store_commit().root_tree.clone())
+        .collect_vec();
+    let new_parent_trees = new_parents
+        .iter()
+        .map(|parent| parent.store_commit().root_tree.clone())
+        .collect_vec();
+    let new_tree_id = if new_parent_trees == old_parent_trees {
+        // Optimization
+        old_commit.tree_id().clone()
+    } else {
+        let old_base_tree = merge_commit_trees(mut_repo.as_repo_ref(), &old_parents);
+        let new_base_tree = merge_commit_trees(mut_repo.as_repo_ref(), new_parents);
+        // TODO: pass in labels for the merge parts
+        merge_trees(&new_base_tree, &old_base_tree, &old_commit.tree()).unwrap()
+    };
     let new_parent_ids = new_parents
         .iter()
         .map(|commit| commit.id().clone())