Support refs to trees (#1449)

* Support refs to trees

* Add test

* Add fallback

* style

* fixup

crates/prek/src/git.rs

@@ -117,15 +117,31 @@ pub(crate) async fn get_changed_files(
     new: &str,
     root: &Path,
 ) -> Result<Vec<PathBuf>, Error> {
-    let output = git_cmd("get changed files")?
-        .arg("diff")
-        .arg("--name-only")
-        .arg("--diff-filter=ACMRT")
-        .arg("--no-ext-diff") // Disable external diff drivers
-        .arg("-z") // Use NUL as line terminator
-        .arg(format!("{old}...{new}"))
-        .arg("--")
-        .arg(root)
+    let build_cmd = |range: String| -> Result<Cmd, Error> {
+        let mut cmd = git_cmd("get changed files")?;
+        cmd.arg("diff")
+            .arg("--name-only")
+            .arg("--diff-filter=ACMRT")
+            .arg("--no-ext-diff") // Disable external diff drivers
+            .arg("-z") // Use NUL as line terminator
+            .arg(range)
+            .arg("--")
+            .arg(root);
+        Ok(cmd)
+    };
+
+    // Try three-dot syntax first (merge-base diff), which works for commits
+    let output = build_cmd(format!("{old}...{new}"))?
+        .check(false)
+        .output()
+        .await?;
+
+    if output.status.success() {
+        return Ok(zsplit(&output.stdout)?);
+    }
+
+    // Fall back to two-dot syntax, which works with both commits and trees
+    let output = build_cmd(format!("{old}..{new}"))?
         .check(true)
         .output()
         .await?;

crates/prek/tests/run.rs

@@ -2932,3 +2932,55 @@ fn expands_tilde_in_prek_home() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn run_with_tree_object_as_ref() -> Result<()> {
+    let context = TestContext::new();
+    context.init_project();
+    context.configure_git_author();
+
+    let cwd = context.work_dir();
+    context.write_pre_commit_config(indoc::indoc! {r"
+        repos:
+          - repo: local
+            hooks:
+              - id: echo-files
+                name: echo files
+                entry: echo
+                language: system
+                pass_filenames: true
+    "});
+
+    // Create initial commit
+    cwd.child("file1.txt").write_str("hello")?;
+    context.git_add(".");
+    context.git_commit("Initial commit");
+
+    // Create some changes and stage them
+    cwd.child("file2.txt").write_str("world")?;
+    context.git_add("file2.txt");
+
+    // Get the tree object from the staged changes
+    let tree_output = Command::new("git")
+        .arg("write-tree")
+        .current_dir(cwd)
+        .output()
+        .expect("Failed to run git write-tree");
+    let tree_sha = String::from_utf8_lossy(&tree_output.stdout)
+        .trim()
+        .to_string();
+
+    // Run prek with tree object as to-ref (should work with .. syntax)
+    cmd_snapshot!(context.filters(), context.run()
+        .arg("--from-ref").arg("HEAD")
+        .arg("--to-ref").arg(&tree_sha), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    echo files...............................................................Passed
+
+    ----- stderr -----
+    ");
+
+    Ok(())
+}