cli: disallow initializing repo with native backend by default

The native backend is just a proof of concept and there's no real
reason to use it other than for testing, so let's reduce the risk of
accidentally creating repos using it.

lib/src/settings.rs

@@ -100,6 +100,12 @@ impl UserSettings {
             .unwrap_or(false)
     }
 
+    pub fn allow_native_backend(&self) -> bool {
+        self.config
+            .get_bool("ui.allow-init-native")
+            .unwrap_or(false)
+    }
+
     pub fn config(&self) -> &config::Config {
         &self.config
     }

src/commands.rs

@@ -1097,6 +1097,13 @@ fn cmd_init(ui: &mut Ui, command: &CommandHelper, args: &InitArgs) -> Result<(),
     } else if args.git {
         Workspace::init_internal_git(ui.settings(), &wc_path)?;
     } else {
+        if !ui.settings().allow_native_backend() {
+            return Err(CommandError::UserError(
+                "The native backend is disallowed by default. Did you mean to pass `--git`?
+Set `ui.allow-init-native` to allow initializing a repo with the native backend."
+                    .to_string(),
+            ));
+        }
         Workspace::init_local(ui.settings(), &wc_path)?;
     };
     let cwd = ui.cwd().canonicalize().unwrap();

tests/test_init_command.rs

@@ -143,9 +143,24 @@ fn test_init_git_colocated() {
     "###);
 }
 
+#[test]
+fn test_init_local_disallowed() {
+    let test_env = TestEnvironment::default();
+    let stdout = test_env.jj_cmd_failure(test_env.env_root(), &["init", "repo"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Error: The native backend is disallowed by default. Did you mean to pass `--git`?
+    Set `ui.allow-init-native` to allow initializing a repo with the native backend.
+    "###);
+}
+
 #[test]
 fn test_init_local() {
     let test_env = TestEnvironment::default();
+    test_env.add_config(
+        br#"[ui]
+    allow-init-native = true
+    "#,
+    );
     let stdout = test_env.jj_cmd_success(test_env.env_root(), &["init", "repo"]);
     insta::assert_snapshot!(stdout, @r###"
     Initialized repo in "repo"

tests/test_untrack_command.rs

@@ -21,6 +21,11 @@ pub mod common;
 #[test]
 fn test_untrack() {
     let test_env = TestEnvironment::default();
+    test_env.add_config(
+        br#"[ui]
+    allow-init-native = true
+    "#,
+    );
     test_env.jj_cmd_success(test_env.env_root(), &["init", "repo"]);
     let repo_path = test_env.env_root().join("repo");
 
@@ -88,6 +93,11 @@ fn test_untrack() {
 #[test]
 fn test_untrack_sparse() {
     let test_env = TestEnvironment::default();
+    test_env.add_config(
+        br#"[ui]
+    allow-init-native = true
+    "#,
+    );
     test_env.jj_cmd_success(test_env.env_root(), &["init", "repo"]);
     let repo_path = test_env.env_root().join("repo");