split: keep change id with child if --empty-parent is used

lib/src/commit_builder.rs

@@ -119,6 +119,14 @@ impl CommitBuilder<'_> {
         self
     }
 
+    pub fn apply_if<F: FnOnce(Self) -> Self>(self, condition: bool, f: F) -> Self {
+        if condition {
+            f(self)
+        } else {
+            self
+        }
+    }
+
     pub fn write(self) -> BackendResult<Commit> {
         let mut rewrite_source_id = None;
         if let Some(rewrite_source) = self.rewrite_source {

src/commands/mod.rs

@@ -29,6 +29,7 @@ use indexmap::{IndexMap, IndexSet};
 use itertools::Itertools;
 use jujutsu_lib::backend::{CommitId, ObjectId, TreeValue};
 use jujutsu_lib::commit::Commit;
+use jujutsu_lib::commit_builder::CommitBuilder;
 use jujutsu_lib::dag_walk::topo_order_reverse;
 use jujutsu_lib::index::IndexEntry;
 use jujutsu_lib::matchers::{EverythingMatcher, NothingMatcher};
@@ -2638,14 +2639,15 @@ don't make any changes, then the operation will be aborted.
         .mut_repo()
         .rewrite_commit(command.settings(), &commit)
         .set_tree(tree_id)
+        .apply_if(args.empty_parent, CommitBuilder::generate_new_change_id)
         .set_description(first_description)
         .write()?;
     let second_commit = tx
         .mut_repo()
         .rewrite_commit(command.settings(), &commit)
         .set_parents(vec![first_commit.id().clone()])
         .set_tree(commit.tree_id().clone())
-        .generate_new_change_id()
+        .apply_if(!args.empty_parent, CommitBuilder::generate_new_change_id)
         .set_description(second_description)
         .write()?;
     let mut rebaser = DescendantRebaser::new(

tests/test_split_command.rs

@@ -254,8 +254,8 @@ fn test_split_empty_parent() {
     @ d043564ef936 second original child
     | o ffdaa62087a2 first original child
     |/  
-    o 19b790168e73 target commit
-    o 8e4fac809cbb (empty) (no description set)
+    o 8e4fac809cbb target commit
+    o 19b790168e73 (empty) (no description set)
     o 9a45c67d3e96 original parent
     o 000000000000 (empty) (no description set)
     "###);