store: add function to look up MergedTree by MergedTreeId

We'll start seeing `MergedTreeId` in more places and we'll want it to
be easy to look up the tree.

lib/src/commit.rs

@@ -110,16 +110,7 @@ impl Commit {
     }
 
     pub fn merged_tree(&self) -> Result<MergedTree, BackendError> {
-        match &self.data.root_tree {
+        self.store.get_root_tree(&self.data.root_tree)
-            MergedTreeId::Legacy(id) => {
-                let tree = self.store.get_tree(&RepoPath::root(), id)?;
-                Ok(MergedTree::Legacy(tree))
-            }
-            MergedTreeId::Merge(ids) => {
-                let trees = ids.try_map(|id| self.store.get_tree(&RepoPath::root(), id))?;
-                Ok(MergedTree::Merge(trees))
-            }
-        }
     }
 
     // TODO(#1624): delete when all callers have been updated to support tree-level

lib/src/store.rs

@@ -21,10 +21,12 @@ use std::sync::{Arc, RwLock};
 
 use crate::backend;
 use crate::backend::{
-    Backend, BackendResult, ChangeId, CommitId, ConflictId, FileId, SymlinkId, TreeId, TreeValue,
+    Backend, BackendResult, ChangeId, CommitId, ConflictId, FileId, MergedTreeId, SymlinkId,
+    TreeId, TreeValue,
 };
 use crate::commit::Commit;
 use crate::merge::Merge;
+use crate::merged_tree::MergedTree;
 use crate::repo_path::RepoPath;
 use crate::tree::Tree;
 use crate::tree_builder::TreeBuilder;
@@ -125,6 +127,19 @@ impl Store {
         Ok(data)
     }
 
+    pub fn get_root_tree(self: &Arc<Self>, id: &MergedTreeId) -> BackendResult<MergedTree> {
+        match &id {
+            MergedTreeId::Legacy(id) => {
+                let tree = self.get_tree(&RepoPath::root(), id)?;
+                Ok(MergedTree::Legacy(tree))
+            }
+            MergedTreeId::Merge(ids) => {
+                let trees = ids.try_map(|id| self.get_tree(&RepoPath::root(), id))?;
+                Ok(MergedTree::Merge(trees))
+            }
+        }
+    }
+
     pub fn write_tree(
         self: &Arc<Self>,
         path: &RepoPath,