feat: implement --new flag

The new `--new` or `-n` flag allows to create a new revision after the
single revision being described.

Running `jj-cz --new` is the equivalent of running `jj-cz --new @`,
describing the current revision and creating a new revision after it.

Passing several revisions to `jj-cz` with the `--new` flag will result
in an error.

tests/error_tests.rs

@@ -135,3 +135,155 @@ fn test_jj_operation_context() {
         panic!("Expected JjOperation variant");
     }
 }
+
+/// Test conversion from std::io::Error
+#[test]
+fn test_from_io_error() {
+    let io_err = std::io::Error::new(std::io::ErrorKind::NotFound, "file not found");
+    let error: Error = io_err.into();
+    assert!(matches!(error, Error::FailedGettingCurrentDir));
+}
+
+/// Test conversion from std::sync::PoisonError
+#[test]
+fn test_from_poison_error() {
+    let mutex = std::sync::Mutex::new(());
+    // Poison the mutex by panicking while holding the lock
+    let poison_err = std::panic::catch_unwind(|| {
+        let _guard = mutex.lock().unwrap();
+        panic!("deliberate panic");
+    });
+    assert!(poison_err.is_err());
+
+    // Now lock should fail with PoisonError
+    let result = mutex.lock();
+    assert!(result.is_err());
+
+    let error: Error = result.unwrap_err().into();
+    assert!(matches!(error, Error::JjOperation { .. }));
+    assert_eq!(
+        format!("{}", error),
+        "Repository operation failed: internal lock poisoned"
+    );
+}
+
+/// Test from_revset_evaluation_error constructs RevsetResolutionError
+#[test]
+fn test_from_revset_evaluation_error() {
+    let underlying = std::io::Error::new(std::io::ErrorKind::Other, "store failure");
+    let eval_err = jj_lib::revset::RevsetEvaluationError::Other(Box::new(underlying));
+    let error = Error::from_revset_evaluation_error("@", eval_err);
+    assert!(matches!(error, Error::RevsetResolutionError { .. }));
+    let description = format!("{}", error);
+    assert!(description.contains("@"));
+    assert!(description.contains("store failure"));
+}
+
+/// Test from_revset_resolution_error constructs RevsetResolutionError
+#[test]
+fn test_from_revset_resolution_error() {
+    let resolution_err = jj_lib::revset::RevsetResolutionError::NoSuchRevision {
+        name: "nonexistent".to_string(),
+        candidates: Vec::new(),
+    };
+    let error = Error::from_revset_resolution_error("@", resolution_err);
+    assert!(matches!(error, Error::RevsetResolutionError { .. }));
+    let description = format!("{}", error);
+    assert!(description.contains("@"));
+    assert!(description.contains("nonexistent"));
+}
+
+/// Test NewFlagWithMultipleRevisions error display
+#[test]
+fn test_new_flag_with_multiple_revisions() {
+    let error = Error::NewFlagWithMultipleRevisions;
+    assert_eq!(
+        format!("{}", error),
+        "--new cannot be used with multiple revisions"
+    );
+}
+
+/// Test NonInteractive error display
+#[test]
+fn test_non_interactive() {
+    let error = Error::NonInteractive;
+    assert_eq!(format!("{}", error), "Non-interactive terminal detected");
+}
+
+/// Test FailedReadingConfig error display
+#[test]
+fn test_failed_reading_config() {
+    let error = Error::FailedReadingConfig {
+        context: "config parse error".to_string(),
+    };
+    let description = format!("{}", error);
+    assert!(description.contains("config parse error"));
+}
+
+/// Test MultipleRevisions error display
+#[test]
+fn test_multiple_revisions() {
+    let error = Error::MultipleRevisions {
+        revset: "abc | def".to_string(),
+    };
+    let description = format!("{}", error);
+    assert!(description.contains("abc | def"));
+    assert!(description.contains("multiple commits"));
+}
+
+/// Test RepositoryLocked error display
+#[test]
+fn test_repository_locked() {
+    let error = Error::RepositoryLocked;
+    assert_eq!(
+        format!("{}", error),
+        "Repository is locked by another process"
+    );
+}
+
+/// Test FailedGettingCurrentDir error display
+#[test]
+fn test_failed_getting_current_dir() {
+    let error = Error::FailedGettingCurrentDir;
+    assert_eq!(format!("{}", error), "Could not get current directory");
+}
+
+/// Test error matching on all variants
+#[test]
+fn test_error_matching_all_variants() {
+    let variants: Vec<Error> = vec![
+        Error::InvalidScope("s".into()),
+        Error::InvalidDescription("d".into()),
+        Error::InvalidCommitMessage("m".into()),
+        Error::NotARepository,
+        Error::JjOperation {
+            context: "c".into(),
+        },
+        Error::RepositoryLocked,
+        Error::FailedGettingCurrentDir,
+        Error::FailedReadingConfig {
+            context: "c".into(),
+        },
+        Error::Cancelled,
+        Error::NonInteractive,
+        Error::RevsetResolutionError {
+            revset: "@".into(),
+            context: "c".into(),
+        },
+        Error::MultipleRevisions { revset: "@".into() },
+        Error::NewFlagWithMultipleRevisions,
+    ];
+
+    // All variants should be displayable without panicking
+    for variant in &variants {
+        let _ = format!("{}", variant);
+        let _ = format!("{:?}", variant);
+    }
+
+    // Verify all variants can be cloned
+    let cloned: Vec<Error> = variants.iter().map(|e| e.clone()).collect();
+    assert_eq!(variants.len(), cloned.len());
+    for (original, clone) in variants.iter().zip(cloned.iter()) {
+        assert_eq!(original, clone);
+    }
+}