examples: show how to add custom CLI command

It's now pretty easy to add a custom command. Let's have an example to
show how to do it, and to protect against regressions.

It would be nice to also have some tests of the custom command, to
check that the command works, that other commands can be run, and that
`jj debug completion` includes the custom command. I'm not sure how to
do that, however.

examples/custom-command/main.rs

@@ -0,0 +1,68 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use clap::{FromArgMatches, Subcommand};
+use jujutsu::cli_util::{
+    create_ui, parse_args, report_command_error, short_commit_description, CommandError,
+};
+use jujutsu::commands::{default_app, run_command};
+use jujutsu::ui::Ui;
+use jujutsu_lib::commit_builder::CommitBuilder;
+
+#[derive(clap::Parser, Clone, Debug)]
+enum CustomCommands {
+    Frobnicate(FrobnicateArgs),
+}
+
+/// Frobnicate a revisions
+#[derive(clap::Args, Clone, Debug)]
+struct FrobnicateArgs {
+    /// The revision to frobnicate
+    #[clap(default_value = "@")]
+    revision: String,
+}
+
+fn run(ui: &mut Ui) -> Result<(), CommandError> {
+    let app = CustomCommands::augment_subcommands(default_app());
+    let (command_helper, matches) = parse_args(ui, app, std::env::args_os())?;
+    match CustomCommands::from_arg_matches(&matches) {
+        // Handle our custom command
+        Ok(CustomCommands::Frobnicate(args)) => {
+            let mut workspace_command = command_helper.workspace_helper(ui)?;
+            let commit = workspace_command.resolve_single_rev(&args.revision)?;
+            let mut tx = workspace_command.start_transaction("Frobnicate");
+            let new_commit = CommitBuilder::for_rewrite_from(ui.settings(), &commit)
+                .set_description("Frobnicated!".to_string())
+                .write_to_repo(tx.mut_repo());
+            workspace_command.finish_transaction(ui, tx)?;
+            writeln!(
+                ui,
+                "Frobnicated revision: {}",
+                short_commit_description(&new_commit)
+            )?;
+            Ok(())
+        }
+        // Handle default commands
+        Err(_) => run_command(ui, &command_helper, &matches),
+    }
+}
+
+fn main() {
+    let mut ui = create_ui();
+    let exit_code = match run(&mut ui) {
+        Ok(()) => 0,
+        Err(err) => report_command_error(&mut ui, err),
+    };
+    std::process::exit(exit_code);
+}