cli: add `jj clone` alias that errors with a friendly message but can be overridden

The main goal here is so that a completely new user that types in `jj
clone` 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.
+ * The `jj init` stub that prints an error is now an alias that can be
+  overridden. `jj clone` is now a similar alias that prints an error by default.
 
 ### Fixed bugs
 

cli/src/config/misc.toml

···
 b = ["bookmark"]
 ci = ["commit"]
 unamend = ["unsquash"]
+
+# Disabled but overridable commands
 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.""",
+    "--"
+]
+clone = [
+    "util",
+    "error",
+    "`jj clone` is not defined by default. Perhaps you meant `jj git clone`?",
+    "--hint",
+    """You can configure `aliases.clone=["git", "clone"]` if you always use the Git backend.""",
     "--"
 ]
 

cli/tests/test_util_command.rs

···
     [EOF]
     [exit status: 1]
     "#);
+    let output = test_env.run_jj_in(".", ["clone", "https://example.org/repo"]);
+    insta::assert_snapshot!(output, @r#"
+    ------- stderr -------
+    Error: `jj clone` is not defined by default. Perhaps you meant `jj git clone`?
+    Hint: You can configure `aliases.clone=["git", "clone"]` 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 -------