cli: teach jj new to set initial description with -m

I rarely use `jj new`, so this feature is mostly for use in tests.

CHANGELOG.md

@@ -24,6 +24,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * The [standard `$NO_COLOR` environment variable](https://no-color.org/) is now
   respected.
 
+* `jj new` now lets you specify a description with `--message/-m`.
+
 ## [0.3.3] - 2022-03-16
 
 No changes, only trying to get the automated build to work.

src/commands.rs

@@ -1148,6 +1148,9 @@ struct NewArgs {
     /// out.
     #[clap(default_value = "@")]
     revision: String,
+    /// The change description to use
+    #[clap(long, short, default_value = "")]
+    message: String,
 }
 
 /// Move changes from one revision into another
@@ -2942,7 +2945,8 @@ fn cmd_new(ui: &mut Ui, command: &CommandHelper, args: &NewArgs) -> Result<(), C
         repo.store(),
         parent.id().clone(),
         parent.tree().id().clone(),
-    );
+    )
+    .set_description(args.message.clone());
     let mut tx = workspace_command.start_transaction("new empty commit");
     let mut_repo = tx.mut_repo();
     let new_commit = commit_builder.write_to_repo(mut_repo);

tests/test_new.rs

@@ -0,0 +1,36 @@
+// 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 jujutsu::testutils::{get_stdout_string, TestEnvironment};
+
+#[test]
+fn test_new_with_message() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    std::fs::write(repo_path.join("file"), "contents").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m", "add a file"]);
+    test_env.jj_cmd_success(&repo_path, &["new", "-m", "a new commit"]);
+
+    let assert = test_env
+        .jj_cmd(&repo_path, &["log", "-T", "commit_id \" \" description"])
+        .assert()
+        .success();
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    @ 588665964c5aaf306d9617095c8a9c4447cdb30c a new commit
+    o f0f3ab56bfa927e3a65c2ac9a513693d438e271b add a file
+    o 0000000000000000000000000000000000000000 
+    "###);
+}