cli: add jj describe --reset-author

I think requests to reset the author came up twice in the last week,
so let's just add support for it. I copied git's behavior of resetting
the name, email, and timestamp. The flag name is also from git.

CHANGELOG.md

@@ -55,6 +55,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj debug completion`, `jj debug mangen` and `jj debug config-schema` have
   been moved from `jj debug` to `jj support`.
 
+* `jj describe` now supports `--reset-author` for resetting a commit's author
+  to the configured user.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

src/commands/mod.rs

@@ -395,7 +395,7 @@ struct InterdiffArgs {
     format: DiffFormatArgs,
 }
 
-/// Edit the change description
+/// Update the change description or other metadata
 ///
 /// Starts an editor to let you edit the description of a change. The editor
 /// will be $EDITOR, or `pico` if that's not defined.
@@ -413,6 +413,11 @@ struct DescribeArgs {
     /// Read the change description from stdin
     #[arg(long)]
     stdin: bool,
+    /// Reset the author to the configured user
+    ///
+    /// This resets the author name, email, and timestamp.
+    #[arg(long)]
+    reset_author: bool,
 }
 
 /// Update the description and create a new change on top.
@@ -1806,15 +1811,20 @@ fn cmd_describe(
         let template = description_template_for_commit(&workspace_command, &commit)?;
         edit_description(workspace_command.repo(), &template, command.settings())?
     };
-    if description == *commit.description() {
+    if description == *commit.description() && !args.reset_author {
         ui.write("Nothing changed.\n")?;
     } else {
         let mut tx =
             workspace_command.start_transaction(&format!("describe commit {}", commit.id().hex()));
-        tx.mut_repo()
+        let mut commit_builder = tx
+            .mut_repo()
             .rewrite_commit(command.settings(), &commit)
-            .set_description(description)
+            .set_description(description);
-            .write()?;
+        if args.reset_author {
+            let new_author = commit_builder.committer().clone();
+            commit_builder = commit_builder.set_author(new_author);
+        }
+        commit_builder.write()?;
         tx.finish(ui)?;
     }
     Ok(())

tests/test_describe_command.rs

@@ -141,3 +141,49 @@ fn test_describe() {
         .failure();
     assert!(get_stderr_string(&assert).contains("bad-jj-editor-from-jj-editor-env"));
 }
+
+#[test]
+fn test_describe_author() {
+    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");
+
+    test_env.add_config(
+        r#"[template-aliases]
+'format_signature(signature)' = 'signature.name() ++ " " ++ signature.email() ++ " " ++ signature.timestamp()'"#,
+    );
+    let get_signatures = || {
+        test_env.jj_cmd_success(
+            &repo_path,
+            &[
+                "log",
+                "-r@",
+                "-T",
+                r#"format_signature(author) ++ "\n" ++ format_signature(committer)"#,
+            ],
+        )
+    };
+    insta::assert_snapshot!(get_signatures(), @r###"
+    @  Test User test.user@example.com 2001-02-03 04:05:07.000 +07:00
+    │  Test User test.user@example.com 2001-02-03 04:05:07.000 +07:00
+    ~
+    "###);
+
+    // Reset the author (the committer is always reset)
+    test_env.jj_cmd_success(
+        &repo_path,
+        &[
+            "describe",
+            "--config-toml",
+            r#"user.name = "Ove Ridder"
+            user.email = "ove.ridder@example.com""#,
+            "-m=description",
+            "--reset-author",
+        ],
+    );
+    insta::assert_snapshot!(get_signatures(), @r###"
+    @  Ove Ridder ove.ridder@example.com 2001-02-03 04:05:09.000 +07:00
+    │  Ove Ridder ove.ridder@example.com 2001-02-03 04:05:09.000 +07:00
+    ~
+    "###);
+}