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, and the change id

Author: glehmann

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_bookmark_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_bookmark_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_bookmark_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.

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