cli: don't silently omit root parent by "jj new --insert-before"

Spotted by Benjamin Tan.

cli/src/commands/new.rs

@@ -129,17 +129,11 @@ Please use `jj new 'all:x|y'` instead of `jj new --allow-large-revsets x y`.",
                 short_commit_hash(&commit_id),
             )));
         }
-        let mut new_parents_commits: Vec<Commit> = new_parents
+        let new_parents_commits: Vec<Commit> = new_parents
             .evaluate_programmatic(tx.repo())?
             .iter()
             .commits(tx.repo().store())
             .try_collect()?;
-        // The git backend does not support creating merge commits involving the root
-        // commit.
-        if new_parents_commits.len() > 1 {
-            let root_commit = tx.repo().store().root_commit();
-            new_parents_commits.retain(|c| c != &root_commit);
-        }
         let merged_tree = merge_commit_trees(tx.repo(), &new_parents_commits)?;
         let new_parents_commit_id = new_parents_commits.iter().map(|c| c.id().clone()).collect();
         new_commit = tx

cli/tests/test_new_command.rs

@@ -417,26 +417,10 @@ fn test_new_insert_before_no_root_merge() {
     ◉  root
     "###);
 
-    let (stdout, stderr) =
-        test_env.jj_cmd_ok(&repo_path, &["new", "--insert-before", "-m", "G", "B", "D"]);
-    insta::assert_snapshot!(stdout, @"");
+    let stderr =
+        test_env.jj_cmd_failure(&repo_path, &["new", "--insert-before", "-m", "G", "B", "D"]);
     insta::assert_snapshot!(stderr, @r###"
-    Rebased 4 descendant commits
-    Working copy now at: kxryzmor bf9fc493 (empty) G
-    Parent commit      : qpvuntsm 65b1ef43 A | (empty) A
-    "###);
-    insta::assert_snapshot!(get_short_log_output(&test_env, &repo_path), @r###"
-    ◉    F
-    ├─╮
-    │ ◉  E
-    ◉ │  D
-    │ │ ◉  C
-    │ │ ◉  B
-    ├───╯
-    @ │  G
-    ◉ │  A
-    ├─╯
-    ◉  root
+    Error: The Git backend does not support creating merge commits with the root commit as one of the parents.
     "###);
 }