revset: fix crash on "log --at-op 00000000 -r 'root()'"

Spotted while refactoring IdPrefixContext.

lib/src/revset.rs

@@ -1798,7 +1798,18 @@ fn resolve_commit_ref(
             Ok(wc_commits)
         }
         RevsetCommitRef::VisibleHeads => Ok(repo.view().heads().iter().cloned().collect_vec()),
-        RevsetCommitRef::Root => Ok(vec![repo.store().root_commit_id().clone()]),
+        RevsetCommitRef::Root => {
+            let commit_id = repo.store().root_commit_id();
+            if repo.index().has_id(commit_id) {
+                Ok(vec![commit_id.clone()])
+            } else {
+                // The root commit doesn't exist at the root operation.
+                Err(RevsetResolutionError::NoSuchRevision {
+                    name: "root()".to_owned(),
+                    candidates: vec![],
+                })
+            }
+        }
         RevsetCommitRef::Bookmarks(pattern) => {
             let commit_ids = repo
                 .view()

lib/tests/test_revset.rs

@@ -34,6 +34,7 @@ use jj_lib::op_store::RefTarget;
 use jj_lib::op_store::RemoteRef;
 use jj_lib::op_store::RemoteRefState;
 use jj_lib::op_store::WorkspaceId;
+use jj_lib::operation::Operation;
 use jj_lib::repo::Repo;
 use jj_lib::repo_path::RepoPath;
 use jj_lib::repo_path::RepoPathUiConverter;
@@ -942,6 +943,14 @@ fn test_evaluate_expression_root_and_checkout() {
     let test_workspace = TestWorkspace::init(&settings);
     let repo = &test_workspace.repo;
 
+    let root_operation = {
+        let op_store = repo.op_store();
+        let id = op_store.root_operation_id();
+        let data = op_store.read_operation(id).unwrap();
+        Operation::new(op_store.clone(), id.clone(), data)
+    };
+    let root_repo = repo.reload_at(&root_operation).unwrap();
+
     let mut tx = repo.start_transaction(&settings);
     let mut_repo = tx.repo_mut();
 
@@ -954,6 +963,14 @@ fn test_evaluate_expression_root_and_checkout() {
         vec![root_commit.id().clone()]
     );
 
+    // but not in the root operation. It might be okay to pretend that the root
+    // commit exists in the root operation, but queries like "root()" shouldn't
+    // panic in any case.
+    assert_matches!(
+        resolve_symbol(root_repo.as_ref(), "root()"),
+        Err(RevsetResolutionError::NoSuchRevision { .. })
+    );
+
     // Can find the current working-copy commit
     mut_repo
         .set_wc_commit(WorkspaceId::default(), commit1.id().clone())