cli: require at least one path for jj untrack

It seems very unlikely that the user would want to untrack all paths
(that's still possible with `jj untrack .`, if they really want to,
and have added all their current paths to the `.gitignore`).

CHANGELOG.md

@@ -15,6 +15,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * Errors are now printed to stderr (they used to be printed to stdout).
 
+* `jj untrack` now requires at least one path (allowing no arguments was a UX
+  bug).
+
 ## [0.4.0] - 2022-04-02
 
 ### Breaking changes

src/commands.rs

@@ -1001,6 +1001,7 @@ struct CheckoutArgs {
 /// Stop tracking specified paths in the working copy
 #[derive(clap::Args, Clone, Debug)]
 struct UntrackArgs {
+    #[clap(required = true, min_values = 1)]
     paths: Vec<String>,
 }
 

tests/test_untrack_command.rs

@@ -38,6 +38,17 @@ fn test_untrack() {
     std::fs::write(repo_path.join(".gitignore"), "*.bak\n").unwrap();
     let files_before = test_env.jj_cmd_success(&repo_path, &["files"]);
 
+    // Errors out when no path is specified
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["untrack"]);
+    insta::assert_snapshot!(stderr.replace("jj.exe", "jj"), @r###"
+    error: The following required arguments were not provided:
+        <PATHS>...
+
+    USAGE:
+        jj untrack [OPTIONS] <PATHS>...
+
+    For more information try --help
+    "###);
     // Errors out when a specified file is not ignored
     let stderr = test_env.jj_cmd_failure(&repo_path, &["untrack", "file1", "file1.bak"]);
     insta::assert_snapshot!(stderr, @"Error: 'file1' would be added back because it's not ignored. Make sure it's ignored, \