cli: when splitting change without description, skip it on second part

One use case for `jj split` is when creating a new commit from some of
the changes in the working copy. If there's no description on the
working-copy commit in that case, it seems better to not ask the user
to provide one when they're splitting the commit either.

CHANGELOG.md

@@ -69,6 +69,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `ui.default-description` option, to use when describing changes with an empty
   description.
 
+* `jj split` will now leave the description empty on the second part if the
+  description was empty on the input commit.
+
 ### Fixed bugs
 
 * `jj config set --user` and `jj config edit --user` can now be used outside of any repository.

cli/src/commands/mod.rs

@@ -793,7 +793,12 @@ struct DiffeditArgs {
 /// Edit the right side of the diff until it has the content you want in the
 /// first revision. Once you close the editor, your edited content will replace
 /// the previous revision. The remaining changes will be put in a new revision
-/// on top. You will be asked to enter a change description for each.
+/// on top.
+///
+/// If the change you split had a description, you will be asked to enter a
+/// change description for each commit. If the change did not have a
+/// description, the second part will not get a description, and you will be
+/// asked for a description only for the first part.
 #[derive(clap::Args, Clone, Debug)]
 struct SplitArgs {
     /// The revision to split
@@ -3095,17 +3100,21 @@ don't make any changes, then the operation will be aborted.
         .set_tree(tree_id)
         .set_description(first_description)
         .write()?;
-    let second_template = description_template_for_cmd_split(
-        ui,
-        command.settings(),
-        tx.base_workspace_helper(),
-        "Enter commit description for the second part (child).",
-        commit.description(),
-        &middle_tree,
-        &commit.tree(),
-    )?;
-    let second_description =
-        edit_description(tx.base_repo(), &second_template, command.settings())?;
+    let second_description = if commit.description().is_empty() {
+        // If there was no description before, don't ask for one for the second commit.
+        "".to_string()
+    } else {
+        let second_template = description_template_for_cmd_split(
+            ui,
+            command.settings(),
+            tx.base_workspace_helper(),
+            "Enter commit description for the second part (child).",
+            commit.description(),
+            &middle_tree,
+            &commit.tree(),
+        )?;
+        edit_description(tx.base_repo(), &second_template, command.settings())?
+    };
     let second_commit = tx
         .mut_repo()
         .rewrite_commit(command.settings(), &commit)

cli/tests/test_split_command.rs

@@ -55,16 +55,7 @@ fn test_split_by_paths() {
 
     JJ: Lines starting with "JJ: " (like this one) will be removed.
     "###);
-    insta::assert_snapshot!(
-        std::fs::read_to_string(test_env.env_root().join("editor1")).unwrap(), @r###"
-    JJ: Enter commit description for the second part (child).
-
-    JJ: This commit contains the following changes:
-    JJ:     A file1
-    JJ:     A file3
-
-    JJ: Lines starting with "JJ: " (like this one) will be removed.
-    "###);
+    assert!(!test_env.env_root().join("editor1").exists());
 
     insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
     @  kkmpptxzrspx false
@@ -148,7 +139,14 @@ fn test_split_with_non_empty_description() {
     let edit_script = test_env.set_up_fake_editor();
     std::fs::write(
         edit_script,
-        ["dump editor1", "next invocation\n", "dump editor2"].join("\0"),
+        [
+            "dump editor1",
+            "write\npart 1",
+            "next invocation\n",
+            "dump editor2",
+            "write\npart 2",
+        ]
+        .join("\0"),
     )
     .unwrap();
     test_env.jj_cmd_success(&workspace_path, &["split", "file1"]);
@@ -175,6 +173,11 @@ JJ:     A file2
 JJ: Lines starting with "JJ: " (like this one) will be removed.
 "#
     );
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_path), @r###"
+    @  kkmpptxzrspx false part 2
+    ◉  qpvuntsmwlqt false part 1
+    ◉  zzzzzzzzzzzz true
+    "###);
 }
 
 #[test]
@@ -206,21 +209,15 @@ JJ:     A file1
 JJ: Lines starting with "JJ: " (like this one) will be removed.
 "#
     );
-    assert_eq!(
-        std::fs::read_to_string(test_env.env_root().join("editor2")).unwrap(),
-        r#"JJ: Enter commit description for the second part (child).
-
-
-TESTED=TODO
-JJ: This commit contains the following changes:
-JJ:     A file2
-
-JJ: Lines starting with "JJ: " (like this one) will be removed.
-"#
-    );
+    assert!(!test_env.env_root().join("editor2").exists());
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_path), @r###"
+    @  rlvkpnrzqnoo false
+    ◉  qpvuntsmwlqt false TESTED=TODO
+    ◉  zzzzzzzzzzzz true
+    "###);
 }
 
 fn get_log_output(test_env: &TestEnvironment, cwd: &Path) -> String {
-    let template = r#"change_id.short() ++ " " ++ empty"#;
+    let template = r#"separate(" ", change_id.short(), empty, description)"#;
     test_env.jj_cmd_success(cwd, &["log", "-T", template])
 }