cli: make it allowed to have a branch on the root commit again

Closes #1529.

cli/src/commands/branch.rs

@@ -152,7 +152,6 @@ fn cmd_branch_create(
 
     let target_commit =
         workspace_command.resolve_single_rev(args.revision.as_deref().unwrap_or("@"), ui)?;
-    workspace_command.check_rewritable(&target_commit)?;
     let mut tx = workspace_command.start_transaction(&format!(
         "create {} pointing to commit {}",
         make_branch_term(&branch_names),
@@ -183,7 +182,6 @@ fn cmd_branch_set(
 
     let target_commit =
         workspace_command.resolve_single_rev(args.revision.as_deref().unwrap_or("@"), ui)?;
-    workspace_command.check_rewritable(&target_commit)?;
     if !args.allow_backwards
         && !branch_names.iter().all(|branch_name| {
             is_fast_forward(

cli/tests/test_branch_command.rs

@@ -46,14 +46,20 @@ fn test_branch_multiple_names() {
 }
 
 #[test]
-fn test_branch_forbidden_at_root() {
+fn test_branch_at_root() {
     let test_env = TestEnvironment::default();
     test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
     let repo_path = test_env.env_root().join("repo");
 
-    let stderr = test_env.jj_cmd_failure(&repo_path, &["branch", "create", "fred", "-r=root()"]);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["branch", "create", "fred", "-r=root()"]);
+    insta::assert_snapshot!(stdout, @"");
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["git", "export"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Nothing changed.
+    "###);
     insta::assert_snapshot!(stderr, @r###"
-    Error: Cannot rewrite the root commit
+    Failed to export some branches:
+      fred
     "###);
 }