split: do not prevent all changes from going into the first commit

Let the user select all changes interactively and put them into
the first commit, and create a second commit with the possibility
of preserving the current commit message. This was previously only
possible in non-interactive mode by specifying matching paths, e.g.
".".  In both cases, a warning will be issued indicating that the second
commit is empty.

CHANGELOG.md

@@ -91,6 +91,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * `jj git fetch -b <remote-git-branch-name>` will now warn if the branch(es)
    can not be found in any of the specified/configured remotes.
 
+* `jj split` now lets the user select all changes in interactive mode. This may be used
+  to keeping all changes into the first commit while keeping the current commit
+  description for the second commit (the newly created empty one).
+
 ### Fixed bugs
 
 * Update working copy before reporting changes. This prevents errors during reporting

cli/src/commands/split.rs

@@ -99,8 +99,7 @@ You are splitting a commit into two: {}
 The diff initially shows the changes in the commit you're splitting.
 
 Adjust the right side until it shows the contents you want for the first commit.
-The remainder will be in the second commit. If you don't make any changes, then
-the operation will be aborted.
+The remainder will be in the second commit.
 ",
             tx.format_commit_summary(&commit)
         )
@@ -109,12 +108,13 @@ the operation will be aborted.
     // Prompt the user to select the changes they want for the first commit.
     let selected_tree_id =
         diff_selector.select(&base_tree, &end_tree, matcher.as_ref(), format_instructions)?;
-    if &selected_tree_id == commit.tree_id() && diff_selector.is_interactive() {
+    if &selected_tree_id == commit.tree_id() {
         // The user selected everything from the original commit.
-        writeln!(ui.status(), "Nothing changed.")?;
-        return Ok(());
-    }
-    if selected_tree_id == base_tree.id() {
+        writeln!(
+            ui.warning_default(),
+            "All changes have been selected, so the second commit will be empty"
+        )?;
+    } else if selected_tree_id == base_tree.id() {
         // The user selected nothing, so the first commit will be empty.
         writeln!(
             ui.warning_default(),

cli/tests/test_split_command.rs

@@ -102,13 +102,14 @@ fn test_split_by_paths() {
     test_env.set_up_fake_editor();
     let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["split", "-r", "@-", "."]);
     insta::assert_snapshot!(stdout, @"");
-    insta::assert_snapshot!(stderr, @r###"
+    insta::assert_snapshot!(stderr, @r#"
+    Warning: All changes have been selected, so the second commit will be empty
     Rebased 1 descendant commits
     First part: qpvuntsm 9da0eea0 (no description set)
     Second part: znkkpsqq 5b5714a3 (empty) (no description set)
     Working copy now at: zsuskuln 0c798ee7 (no description set)
     Parent commit      : znkkpsqq 5b5714a3 (empty) (no description set)
-    "###);
+    "#);
 
     insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
     @  zsuskulnrvyr false
@@ -576,15 +577,14 @@ fn test_split_interactive() {
     let (stdout, stderr) = test_env.jj_cmd_ok(&workspace_path, &["split"]);
 
     insta::assert_snapshot!(
-        std::fs::read_to_string(test_env.env_root().join("instrs")).unwrap(), @r###"
+        std::fs::read_to_string(test_env.env_root().join("instrs")).unwrap(), @r#"
     You are splitting a commit into two: qpvuntsm 44af2155 (no description set)
 
     The diff initially shows the changes in the commit you're splitting.
 
     Adjust the right side until it shows the contents you want for the first commit.
-    The remainder will be in the second commit. If you don't make any changes, then
-    the operation will be aborted.
-    "###);
+    The remainder will be in the second commit.
+    "#);
 
     insta::assert_snapshot!(
         std::fs::read_to_string(test_env.env_root().join("editor")).unwrap(), @r###"