cli `init`: make `jj init` an alias that prints an error

The main goal here is so that a completely new user that types in `jj
init` gets a pointer to the correct command needed to try out `jj`.

CHANGELOG.md

···
 * `jj git fetch` now supports [string pattern syntax](docs/revsets.md#string-patterns)
   on `--remote` option and `git.fetch` configuration.
 
+* The `jj init` stub that prints an error is now an alias that can be overridden.
+
 ### Fixed bugs
 
 * `jj status` now shows untracked files under untracked directories.

cli/src/config/misc.toml

···
 b = ["bookmark"]
 ci = ["commit"]
 unamend = ["unsquash"]
+init = [
+    "util",
+    "error",
+    "`jj init` is not defined by default. Perhaps you meant `jj git init`?",
+    "--hint",
+    """You can configure `aliases.init=["git", "init"]` if you always use the Git backend.""",
+    "--"
+]
 
 [diff.color-words]
 max-inline-alternation = 3

cli/tests/test_util_command.rs

···
     [exit status: 1]
     ");
 }
+
+#[test]
+fn test_erroring_aliases() {
+    let test_env = TestEnvironment::default();
+    let output = test_env.run_jj_in(".", ["init", "repo"]);
+    insta::assert_snapshot!(output, @r#"
+    ------- stderr -------
+    Error: `jj init` is not defined by default. Perhaps you meant `jj git init`?
+    Hint: You can configure `aliases.init=["git", "init"]` if you always use the Git backend.
+    [EOF]
+    [exit status: 1]
+    "#);
+    let output = test_env.run_jj_in(".", ["init", "--help"]);
+    insta::assert_snapshot!(output, @r#"
+    ------- stderr -------
+    Error: `jj init` is not defined by default. Perhaps you meant `jj git init`?
+    Hint: You can configure `aliases.init=["git", "init"]` if you always use the Git backend.
+    [EOF]
+    [exit status: 1]
+    "#);
+
+    // Test that `init` can be overridden as an alias. (We use `jj config get`
+    // as a command with a predictable output)
+    test_env.add_config(r#"aliases.init=["config", "get", "user.name"]"#);
+    let output = test_env.run_jj_in(".", ["init"]);
+    insta::assert_snapshot!(output, @r"
+    Test User
+    [EOF]
+    ");
+}