cli: do not allow jj init --git in existing Git repo

Allowing `jj init --git` in an existing Git repo creates a second Git
store in `.jj/repo/store/git`, totally disconnected from the existing
Git store. This will only produce extremely confusing bugs for users,
since any operations they make in Git will *not* be reflected in the
jj repo.

CHANGELOG.md

@@ -14,6 +14,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * `jj branch set` no longer creates a new branch. Use `jj branch create`
   instead.
+  
+* `jj init --git` in an existing Git repository now errors and exits rather than
+  creating a second Git store.
 
 ### New features
 

cli/src/commands/init.rs

@@ -65,6 +65,8 @@ pub(crate) fn cmd_init(
     let wc_path = wc_path
         .canonicalize()
         .map_err(|e| user_error(format!("Failed to create workspace: {e}")))?; // raced?
+    let cwd = command.cwd().canonicalize().unwrap();
+    let relative_wc_path = file_util::relative_path(&cwd, &wc_path);
 
     if let Some(git_store_str) = &args.git_repo {
         let mut git_store_path = command.cwd().join(git_store_str);
@@ -101,6 +103,16 @@ pub(crate) fn cmd_init(
         }
         print_trackable_remote_branches(ui, workspace_command.repo().view())?;
     } else if args.git {
+        if wc_path.join(".git").exists() {
+            return Err(user_error_with_hint(
+                "Did not create a jj repo because there is an existing Git repo in this directory.",
+                format!(
+                    r#"To create a repo backed by the existing Git repo, run `jj init --git-repo={}` instead."#,
+                    relative_wc_path.display()
+                ),
+            ));
+        }
+
         Workspace::init_internal_git(command.settings(), &wc_path)?;
     } else {
         if !command.settings().allow_native_backend() {
@@ -112,22 +124,12 @@ Set `ui.allow-init-native` to allow initializing a repo with the native backend.
         }
         Workspace::init_local(command.settings(), &wc_path)?;
     };
-    let cwd = command.cwd().canonicalize().unwrap();
-    let relative_wc_path = file_util::relative_path(&cwd, &wc_path);
+
     writeln!(
         ui.stderr(),
         "Initialized repo in \"{}\"",
         relative_wc_path.display()
     )?;
-    if args.git && wc_path.join(".git").exists() {
-        writeln!(ui.warning(), "Empty repo created.")?;
-        writeln!(
-            ui.hint(),
-            "Hint: To create a repo backed by the existing Git repo, run `jj init --git-repo={}` \
-             instead.",
-            relative_wc_path.display()
-        )?;
-    }
     Ok(())
 }
 

cli/tests/test_init_command.rs

@@ -418,16 +418,14 @@ fn test_init_git_external_but_git_dir_exists() {
 }
 
 #[test]
-fn test_init_git_internal_but_could_be_colocated() {
+fn test_init_git_internal_must_be_colocated() {
     let test_env = TestEnvironment::default();
     let workspace_root = test_env.env_root().join("repo");
     init_git_repo(&workspace_root, false);
 
-    let (stdout, stderr) = test_env.jj_cmd_ok(&workspace_root, &["init", "--git"]);
-    insta::assert_snapshot!(stdout, @"");
+    let stderr = test_env.jj_cmd_failure(&workspace_root, &["init", "--git"]);
     insta::assert_snapshot!(stderr, @r###"
-    Initialized repo in "."
-    Empty repo created.
+    Error: Did not create a jj repo because there is an existing Git repo in this directory.
     Hint: To create a repo backed by the existing Git repo, run `jj init --git-repo=.` instead.
     "###);
 }