commands: add --interactive flag to jj split

`jj split` with no arguments operates interactively, but I am nonetheless constantly running `jj split -i` because I expect an `--interactive` flag to exist for consistency.

However, `jj split <paths>` before this commit always operates non-interactively, so this commit has the nice practical effect that you can restrict your interactive splitting to a certain set of paths.

CHANGELOG.md

@@ -33,6 +33,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * jj now bundles a TUI tool to use as the default diff and merge editors. (The
   previous default was `meld`.)
 
+* `jj split` supports the `--interactive` flag. (This is already the default if
+  no paths are provided.)
+
 ### Fixed bugs
 
 ## [0.9.0] - 2023-09-06

cli/src/commands/mod.rs

@@ -877,10 +877,14 @@ struct DiffeditArgs {
 /// asked for a description only for the first part.
 #[derive(clap::Args, Clone, Debug)]
 struct SplitArgs {
+    /// Interactively choose which parts to split. This is the default if no
+    /// paths are provided.
+    #[arg(long, short)]
+    interactive: bool,
     /// The revision to split
     #[arg(long, short, default_value = "@")]
     revision: RevisionArg,
-    /// Put these paths in the first commit and don't run the diff editor
+    /// Put these paths in the first commit
     #[arg(value_hint = clap::ValueHint::AnyPath)]
     paths: Vec<String>,
 }
@@ -3257,7 +3261,7 @@ fn cmd_split(ui: &mut Ui, command: &CommandHelper, args: &SplitArgs) -> Result<(
         workspace_command.start_transaction(&format!("split commit {}", commit.id().hex()));
     let end_tree = commit.tree()?;
     let base_tree = merge_commit_trees(tx.repo(), &commit.parents())?;
-    let interactive = args.paths.is_empty();
+    let interactive = args.interactive || args.paths.is_empty();
     let instructions = format!(
         "\
 You are splitting a commit in two: {}