backout: include backed out commit's subject in new commit

CHANGELOG.md

@@ -30,6 +30,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * `jj workspace forget` now abandons the workspace's working-copy commit if it
   was empty.
 
+* `jj backout` now includes the backed out commit's subject in the new commit
+  message.
+
 ### Fixed bugs
 
 ## [0.19.0] - 2024-07-03
@@ -178,7 +181,6 @@ Thanks to the people who made this release happen!
   were global flags and specifying them once would insert the new commit before/
   after all the specified commits.
 
-
 ### Deprecations
 
 * Attempting to alias a built-in command now gives a warning, rather than being

cli/src/commands/backout.rs

@@ -47,17 +47,24 @@ pub(crate) fn cmd_backout(
         parents.push(destination);
     }
     let mut tx = workspace_command.start_transaction();
+    let commit_to_back_out_subject = commit_to_back_out
+        .description()
+        .lines()
+        .next()
+        .unwrap_or_default();
+    let new_commit_description = format!(
+        "Back out \"{}\"\n\nThis backs out commit {}.\n",
+        commit_to_back_out_subject,
+        &commit_to_back_out.id().hex()
+    );
     let old_base_tree = commit_to_back_out.parent_tree(tx.mut_repo())?;
     let new_base_tree = merge_commit_trees(tx.mut_repo(), &parents)?;
     let old_tree = commit_to_back_out.tree()?;
     let new_tree = new_base_tree.merge(&old_tree, &old_base_tree)?;
     let new_parent_ids = parents.iter().map(|commit| commit.id().clone()).collect();
     tx.mut_repo()
         .new_commit(command.settings(), new_parent_ids, new_tree.id())
-        .set_description(format!(
-            "backout of commit {}",
-            &commit_to_back_out.id().hex()
-        ))
+        .set_description(new_commit_description)
         .write()?;
     tx.finish(
         ui,

cli/tests/test_backout_command.rs

@@ -50,7 +50,9 @@ fn test_backout() {
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @"");
     insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
-    ◉  8fe4e9345020 backout of commit 2443ea76b0b1c531326908326aab7020abab8e6c
+    ◉  6d845ed9fb6a Back out "a"
+    │
+    │  This backs out commit 2443ea76b0b1c531326908326aab7020abab8e6c.
     @  2443ea76b0b1 a
     ◉  000000000000
     "###);
@@ -65,8 +67,12 @@ fn test_backout() {
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @"");
     insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
-    ◉  46f192066e5e backout of commit 8fe4e93450209a70a6b9cd9af612e86634fd31fd
-    @  8fe4e9345020 backout of commit 2443ea76b0b1c531326908326aab7020abab8e6c
+    ◉  79555ea9040b Back out "Back out "a""
+    │
+    │  This backs out commit 6d845ed9fb6a3d367e2d7068ef0256b1a10705a9.
+    @  6d845ed9fb6a Back out "a"
+    │
+    │  This backs out commit 2443ea76b0b1c531326908326aab7020abab8e6c.
     ◉  2443ea76b0b1 a
     ◉  000000000000
     "###);