rebase: add tests of rebasing empty sets

As these tests show, we sometimes print an error when trying to rebase
an empty set, and sometimes we don't say anything at all. It seems to
me like we should say "Nothing changed" in all of these cases.

cli/tests/test_rebase_command.rs

@@ -115,6 +115,29 @@ fn test_rebase_invalid() {
     "###);
 }
 
+#[test]
+fn test_rebase_empty_sets() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    create_commit(&test_env, &repo_path, "a", &[]);
+    create_commit(&test_env, &repo_path, "b", &["a"]);
+
+    // TODO: Make all of these say "Nothing changed"?
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["rebase", "-r=none()", "-d=b"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @"");
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["rebase", "-s=none()", "-d=b"]);
+    insta::assert_snapshot!(stderr, @r###"Error: Revset "none()" didn't resolve to any revisions"###);
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["rebase", "-b=none()", "-d=b"]);
+    insta::assert_snapshot!(stderr, @r###"Error: Revset "none()" didn't resolve to any revisions"###);
+    // Empty because "b..a" is empty
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["rebase", "-b=a", "-d=b"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @"");
+}
+
 #[test]
 fn test_rebase_bookmark() {
     let test_env = TestEnvironment::default();