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

@@ -85,6 +85,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   anywhere in the revision tree with the `--insert-before`, `--insert-after` and
   `--destination` command line flags.
 
+* `jj split` assigns the change id of the source revision to the second revision.
+  You can opt out of this change by setting `split.legacy-change-id-behavior = true`,
+  but this will likely be removed in a future release.
+
 ### Fixed bugs
 
 * Fixed crash on change-delete conflict resolution.

cli/src/commands/split.rs

@@ -221,10 +221,17 @@ 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_change_id_behavior = tx.settings().get_bool("split.legacy-change-id-behavior")?;
+
     // Create the first commit, which includes the changes selected by the user.
     let mut first_commit = {
         let mut commit_builder = tx.repo_mut().rewrite_commit(&target.commit).detach();
         commit_builder.set_tree_id(target.selected_tree.id());
+        if !legacy_change_id_behavior {
+            // Generate a new change id so that the commit being split doesn't
+            // become divergent.
+            commit_builder.generate_new_change_id();
+        }
         let description = if !args.message_paragraphs.is_empty() {
             let description = join_message_paragraphs(&args.message_paragraphs);
             if !description.is_empty() {
@@ -269,10 +276,12 @@ pub(crate) fn cmd_split(
         let mut commit_builder = tx.repo_mut().rewrite_commit(&target.commit).detach();
         commit_builder
             .set_parents(parents)
-            .set_tree_id(new_tree.id())
+            .set_tree_id(new_tree.id());
+        if legacy_change_id_behavior {
             // Generate a new change id so that the commit being split doesn't
             // become divergent.
-            .generate_new_change_id();
+            commit_builder.generate_new_change_id();
+        }
         let description = if target.commit.description().is_empty() {
             // If there was no description before, don't ask for one for the
             // second commit.
@@ -300,6 +309,9 @@ pub(crate) fn cmd_split(
     if legacy_bookmark_behavior {
         tx.repo_mut()
             .set_rewritten_commit(target.commit.id().clone(), second_commit.id().clone());
+    } else {
+        tx.repo_mut()
+            .set_rewritten_commit(target.commit.id().clone(), first_commit.id().clone());
     }
 
     let mut rewritten_commits = HashMap::<CommitId, CommitId>::new();

cli/src/config-schema.json

@@ -785,6 +785,11 @@
                 "legacy-bookmark-behavior": {
                     "type": "boolean",
                     "description": "If true, bookmarks will move to the second commit instead of the first.",
+                    "default": true
+                },
+                "legacy-change-id-behavior": {
+                    "type": "boolean",
+                    "description": "If true, the first commit will get the original change id instead of the second commit.",
                     "default": false
                 }
             }

cli/src/config/misc.toml

@@ -55,3 +55,4 @@ auto-update-stale = false
 # in the future.
 [split]
 legacy-bookmark-behavior = true
+legacy-change-id-behavior = false