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 @ && jj new`. Running `jj-cz --new xs` is
the equivalent of running `jj-cz xs && jj new xs`.

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

Refs: #6

src/cli/mod.rs

@@ -18,6 +18,10 @@ pub struct Cli {
     /// The revision(s) whose description to edit (default: @)
     #[arg(value_name = "REVSETS")]
     revsets: Vec<String>,
+
+    /// Create a new child revision after editing the description
+    #[arg(short, long)]
+    new: bool,
 }
 
 impl Cli {
@@ -29,4 +33,95 @@ impl Cli {
             self.revsets.iter().map(|s| s.as_str()).collect()
         }
     }
+
+    pub fn create_new(&self) -> bool {
+        self.new
+    }
+
+    pub fn validate(&self) -> Result<(), jj_cz::Error> {
+        if self.new && self.revsets().len() > 1 {
+            Err(jj_cz::Error::NewFlagWithMultipleRevisions)
+        } else {
+            Ok(())
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use clap::Parser;
+
+    #[test]
+    fn revsets_defaults_to_at() {
+        let cli = Cli::parse_from(["jj-cz"]);
+        assert_eq!(cli.revsets(), vec!["@"]);
+    }
+
+    #[test]
+    fn revsets_returns_provided_values() {
+        let cli = Cli::parse_from(["jj-cz", "abc", "def"]);
+        let revsets = cli.revsets();
+        assert_eq!(revsets, vec!["abc", "def"]);
+    }
+
+    #[test]
+    fn revsets_single_revset() {
+        let cli = Cli::parse_from(["jj-cz", "xyz"]);
+        assert_eq!(cli.revsets(), vec!["xyz"]);
+    }
+
+    #[test]
+    fn create_new_returns_false_by_default() {
+        let cli = Cli::parse_from(["jj-cz"]);
+        assert!(!cli.create_new());
+    }
+
+    #[test]
+    fn create_new_returns_true_with_flag() {
+        let cli = Cli::parse_from(["jj-cz", "--new"]);
+        assert!(cli.create_new());
+    }
+
+    #[test]
+    fn create_new_returns_true_with_short_flag() {
+        let cli = Cli::parse_from(["jj-cz", "-n"]);
+        assert!(cli.create_new());
+    }
+
+    #[test]
+    fn validate_ok_with_no_args() {
+        let cli = Cli::parse_from(["jj-cz"]);
+        assert!(cli.validate().is_ok());
+    }
+
+    #[test]
+    fn validate_ok_with_new_and_single_revset() {
+        let cli = Cli::parse_from(["jj-cz", "--new", "@"]);
+        assert!(cli.validate().is_ok());
+    }
+
+    #[test]
+    fn validate_ok_with_multiple_revsets_no_new() {
+        let cli = Cli::parse_from(["jj-cz", "abc", "def"]);
+        assert!(cli.validate().is_ok());
+    }
+
+    #[test]
+    fn validate_err_with_new_and_multiple_revsets() {
+        let cli = Cli::parse_from(["jj-cz", "--new", "abc", "def"]);
+        let result = cli.validate();
+        assert!(result.is_err());
+        assert!(matches!(
+            result.unwrap_err(),
+            jj_cz::Error::NewFlagWithMultipleRevisions
+        ));
+    }
+
+    #[test]
+    fn cli_derives_debug() {
+        let cli = Cli::parse_from(["jj-cz", "--new", "@"]);
+        let debug = format!("{:?}", cli);
+        assert!(debug.contains("Cli"));
+    }
 }