split: allow creation of empty parent or child

jj split .           => create an empty child
jj split nonexistent => create an empty parent

CHANGELOG.md

@@ -89,6 +89,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Commands that draw an ASCII graph (`jj log`, `jj op log`, `jj obslog`) now
   have different styles available by setting e.g. `ui.graph.style = "curved"`.
 
+* `jj split` accepts creating empty commits when given a path. `jj split .`
+  inserts an empty commit between the target commit and its children if any,
+  and `jj split any-non-existent-path` inserts an empty commit between the
+  target commit and its parents.
+
 ### Fixed bugs
 
 * When sharing the working copy with a Git repo, we used to forget to export

src/commands/mod.rs

@@ -2557,6 +2557,7 @@ fn cmd_split(ui: &mut Ui, command: &CommandHelper, args: &SplitArgs) -> Result<(
     let mut tx =
         workspace_command.start_transaction(&format!("split commit {}", commit.id().hex()));
     let base_tree = merge_commit_trees(tx.base_repo().as_repo_ref(), &commit.parents());
+    let interactive = args.paths.is_empty();
     let instructions = format!(
         "\
 You are splitting a commit in two: {}
@@ -2574,10 +2575,10 @@ don't make any changes, then the operation will be aborted.
         &base_tree,
         &commit.tree(),
         &instructions,
-        args.paths.is_empty(),
+        interactive,
         matcher.as_ref(),
     )?;
-    if &tree_id == commit.tree_id() {
+    if &tree_id == commit.tree_id() && interactive {
         ui.write("Nothing changed.\n")?;
     } else {
         let middle_tree = tx

tests/test_split_command.rs

@@ -80,4 +80,55 @@ fn test_split_by_paths() {
     A file1
     A file3
     "###);
+
+    // Insert an empty commit after @- with "split ."
+    test_env.set_up_fake_editor();
+    let stdout = test_env.jj_cmd_success(&repo_path, &["split", "-r", "@-", "."]);
+    insta::assert_snapshot!(stdout, @r###"
+    Rebased 1 descendant commits
+    First part: 31425b568fcf (no description set)
+    Second part: af0963926ac3 (no description set)
+    Working copy now at: 28d4ec20efa9 (no description set)
+    "###);
+
+    let stdout =
+        test_env.jj_cmd_success(&repo_path, &["log", "-T", r#"change_id.short() " " empty"#]);
+    insta::assert_snapshot!(stdout, @r###"
+    @ ffdaa62087a2 false
+    o 19b790168e73 true
+    o 9a45c67d3e96 false
+    o 000000000000 true
+    "###);
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s", "-r", "@--"]);
+    insta::assert_snapshot!(stdout, @r###"
+    A file2
+    "###);
+
+    // Remove newly created empty commit
+    test_env.jj_cmd_success(&repo_path, &["abandon", "@-"]);
+
+    // Insert an empty commit before @- with "split nonexistent"
+    test_env.set_up_fake_editor();
+    let stdout = test_env.jj_cmd_success(&repo_path, &["split", "-r", "@-", "nonexistent"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Rebased 1 descendant commits
+    First part: 0647b2cbd0da (no description set)
+    Second part: d5d77af65446 (no description set)
+    Working copy now at: 86f228dc3a50 (no description set)
+    "###);
+
+    let stdout =
+        test_env.jj_cmd_success(&repo_path, &["log", "-T", r#"change_id.short() " " empty"#]);
+    insta::assert_snapshot!(stdout, @r###"
+    @ ffdaa62087a2 false
+    o fa9213bcf78e false
+    o 9a45c67d3e96 true
+    o 000000000000 true
+    "###);
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s", "-r", "@-"]);
+    insta::assert_snapshot!(stdout, @r###"
+    A file2
+    "###);
 }