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

@@ -87,10 +87,14 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * `jj diffedit`, `jj abandon`, and `jj restore` now accept a `--restore-descendants`
   flag. When used, descendants of the edited or deleted commits will keep their original
   content.
-
+  
 * `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, 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, 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