Make jj undo work after jj duplicate

Fixes https://github.com/martinvonz/jj/issues/1050

Thanks to Martin for suggesting the exact fix.

The tests go into the new tests/test_duplicate_command.rs, which will be
expanded shortly with other tests depending on this bugfix.

CHANGELOG.md

@@ -81,6 +81,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Fixed handling of escaped characters in .gitignore (only keep trailing spaces
   if escaped properly).
 
+* `jj undo` now works after `jj duplicate`.
+
 ### Contributors
 
 Thanks to the people who made this release happen!

lib/src/repo.rs

@@ -610,7 +610,9 @@ impl MutableRepo {
     }
 
     pub fn has_changes(&self) -> bool {
-        self.view() != &self.base_repo.view
+        !(self.abandoned_commits.is_empty()
+            && self.rewritten_commits.is_empty()
+            && self.view() == &self.base_repo.view)
     }
 
     pub fn consume(self) -> (MutableIndex, View) {

tests/test_duplicate_command.rs

@@ -0,0 +1,73 @@
+// Copyright 2023 The Jujutsu Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::path::Path;
+
+use crate::common::TestEnvironment;
+
+pub mod common;
+
+fn create_commit(test_env: &TestEnvironment, repo_path: &Path, name: &str, parents: &[&str]) {
+    if parents.is_empty() {
+        test_env.jj_cmd_success(repo_path, &["new", "root", "-m", name]);
+    } else {
+        let mut args = vec!["new", "-m", name];
+        args.extend(parents);
+        test_env.jj_cmd_success(repo_path, &args);
+    }
+    std::fs::write(repo_path.join(name), format!("{name}\n")).unwrap();
+    test_env.jj_cmd_success(repo_path, &["branch", "create", name]);
+}
+
+// https://github.com/martinvonz/jj/issues/1050
+#[test]
+fn test_undo_after_duplicate() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    create_commit(&test_env, &repo_path, "a", &[]);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @ 2443ea76b0b1   a
+    o 000000000000   (no description set)
+    "###);
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["duplicate", "a"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Created: f5cefcbb65a4 a
+    "###);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    o f5cefcbb65a4   a
+    | @ 2443ea76b0b1   a
+    |/  
+    o 000000000000   (no description set)
+    "###);
+
+    insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["undo"]), @"");
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @ 2443ea76b0b1   a
+    o 000000000000   (no description set)
+    "###);
+}
+
+fn get_log_output(test_env: &TestEnvironment, repo_path: &Path) -> String {
+    test_env.jj_cmd_success(
+        repo_path,
+        &[
+            "log",
+            "-T",
+            r#"commit_id.short() "   " description.first_line()"#,
+        ],
+    )
+}