split: the second commit keeps the change id

The first commit contains the split out changes.
The second commit usually keeps most of the changes, the change id and
the attached bookmarks (as long as split.legacy-bookmark-behavior is
true, which should stay the default value).

This no-implicit-move behavior aligns with other jj commands.

With the -A/-B/-d options, the split-out commit may be moved anywhere.
With commit tree

    @  pmpklomk (empty) (no description set)
    ○  pzskstlk *letters* fileB
    ○  kmvzrqtu fileA
    │ ○  lyzvpymy *numbers* file3&C
    │ ○  kpmxxkwk file2
    │ ○  xzvtlrrt file1
    ├─╯
    ◆  zzzzzzzz root()

when splitting `fileC` out of `lyzvpymy` to the other branch with

    jj split -r lyzvpymy -d pzskstlk

the change id and the bookmark stay in the branch with the source commit.

    @  pmpklomk (empty) (no description set)
    ○  vowztxqo fileC
    ○  pzskstlk *letters* fileB
    ○  kmvzrqtu fileA
    │ ○  lyzvpymy *numbers* file3
    │ ○  kpmxxkwk file2
    │ ○  xzvtlrrt file1
    ├─╯
    ◆  zzzzzzzz root()

Author: glehmann

CHANGELOG.md

@@ -43,6 +43,11 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * Added `git.track-default-bookmark-on-clone` setting to control whether to
   track the default remote bookmark on `jj git clone`.
 
+* `jj split` assigns the change id and the bookmarks of the source revision
+  to the second revision. 
+  You can opt out of this change by setting `split.legacy-bookmark-behavior = true`,
+  but this will likely be removed in a future release.
+
 ### Fixed bugs
 
 * Work around a git issue that could cause subprocess operations to hang if the

cli/src/commands/split.rs

@@ -49,8 +49,9 @@ use crate::ui::Ui;
 ///
 /// Starts a [diff editor] on the changes in the revision. Edit the right side
 /// of the diff until it has the content you want in the new 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 close the editor, your edited content will be put in a new revision
+/// before the original revision, while the remaining changes will replace the
+/// original revision.
 ///
 /// [diff editor]:
 ///     https://jj-vcs.github.io/jj/latest/config/#editing-diffs
@@ -219,11 +220,14 @@ pub(crate) fn cmd_split(
     // Prompt the user to select the changes they want for the first commit.
     let target = select_diff(ui, &tx, &target_commit, &matcher, &diff_selector)?;
 
+    let legacy_bookmark_behavior =
+        !use_move_flags && tx.settings().get_bool("split.legacy-bookmark-behavior")?;
+
     // Create the first commit, which includes the changes selected by the user.
     let first_commit = {
         let mut commit_builder = tx.repo_mut().rewrite_commit(&target.commit).detach();
         commit_builder.set_tree_id(target.selected_tree.id());
-        if use_move_flags {
+        if !legacy_bookmark_behavior {
             commit_builder
                 // Generate a new change id so that the commit being split doesn't
                 // become divergent.
@@ -274,7 +278,7 @@ pub(crate) fn cmd_split(
         commit_builder
             .set_parents(parents)
             .set_tree_id(new_tree.id());
-        if !use_move_flags {
+        if legacy_bookmark_behavior {
             commit_builder
                 // Generate a new change id so that the commit being split doesn't
                 // become divergent.
@@ -415,14 +419,9 @@ fn rewrite_descendants(
     tx.repo_mut()
         .transform_descendants(vec![target.commit.id().clone()], |mut rewriter| {
             num_rebased += 1;
-            if parallel && legacy_bookmark_behavior {
-                // The old_parent is the second commit due to the rewrite above.
+            if parallel {
                 rewriter
                     .replace_parent(second_commit.id(), [first_commit.id(), second_commit.id()]);
-            } else if parallel {
-                rewriter.replace_parent(first_commit.id(), [first_commit.id(), second_commit.id()]);
-            } else {
-                rewriter.replace_parent(first_commit.id(), [second_commit.id()]);
             }
             rewriter.rebase()?.write()?;
             Ok(())

cli/src/config/misc.toml

@@ -55,4 +55,4 @@ auto-update-stale = false
 # The behavior when this flag is set to false is experimental and may be changed
 # in the future.
 [split]
-legacy-bookmark-behavior = true
+legacy-bookmark-behavior = false

cli/tests/[email protected]

@@ -2442,7 +2442,7 @@ For example, if all you need is the `README.md` and the `lib/` directory, use `j
 
 Split a revision in two
 
-Starts a [diff editor] on the changes in the revision. Edit the right side of the diff until it has the content you want in the new 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.
+Starts a [diff editor] on the changes in the revision. Edit the right side of the diff until it has the content you want in the new revision. Once you close the editor, your edited content will be put in a new revision before the original revision, while the remaining changes will replace the original revision.
 
 [diff editor]: https://jj-vcs.github.io/jj/latest/config/#editing-diffs