cli: add ui.signed-off-by = true to configure sign offs

Many of the desired features for the default commit template, notably
pre-populating things, can be solved with a simple pattern:

1. Inside `template-aliases.draft_commit_message`, use the following:

       if(config("name.space").as_boolean() && !description.contains("..."), ...)`

2. Set `name.space = true` in repos where you want it enabled.

This patch uses this idea to configure a new behavior to get
`Signed-off-by` lines included in their repositories.

While `Signed-off-by:` is a git mechanism, it is a common one and this
makes it easier to use jj OOTB with repositories that might require
those, without adding any more real surface area to the Rust codebase.
This general idea can be configured by the user though, e.g. for
`BUG=123` templates, or Gerrit `Change-Id`s, or other various things.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

cli/src/config/misc.toml

@@ -33,6 +33,7 @@ quiet = false
 log-word-wrap = false
 log-synthetic-elided-nodes = true
 conflict-marker-style = "diff"
+signed-off-by = false
 
 [ui.movement]
 edit = false

cli/src/config/templates.toml

@@ -32,6 +32,10 @@ if(overridden,
 draft_commit_description = '''
 concat(
   description,
+  if(
+    config("ui.signed-off-by").as_boolean() && !description.contains("Signed-off-by: " ++ author.name()),
+    "\nSigned-off-by: " ++ author,
+  ),
   surround(
     "\nJJ: This commit contains the following changes:\n", "",
     indent("JJ:     ", diff.summary()),

cli/tests/test_describe_command.rs

@@ -580,6 +580,37 @@ fn test_describe_default_description() {
     "###);
 }
 
+#[test]
+fn test_describe_signed_off() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    test_env.add_config(r#"ui.default-description = "\n\nTESTED=TODO""#);
+    test_env.add_config(r#"ui.signed-off-by = true"#);
+    let workspace_path = test_env.env_root().join("repo");
+
+    std::fs::write(workspace_path.join("file1"), "foo\n").unwrap();
+    std::fs::write(workspace_path.join("file2"), "bar\n").unwrap();
+    let edit_script = test_env.set_up_fake_editor();
+    std::fs::write(edit_script, ["dump editor"].join("\0")).unwrap();
+    let (stdout, stderr) = test_env.jj_cmd_ok(&workspace_path, &["describe"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r#"
+    Working copy now at: qpvuntsm 9b7a94ea TESTED=TODO
+    Parent commit      : zzzzzzzz 00000000 (empty) (no description set)
+    "#);
+    insta::assert_snapshot!(
+        std::fs::read_to_string(test_env.env_root().join("editor")).unwrap(), @r#"
+    TESTED=TODO
+
+    Signed-off-by: Test User <test.user@example.com>
+    JJ: This commit contains the following changes:
+    JJ:     A file1
+    JJ:     A file2
+
+    JJ: Lines starting with "JJ:" (like this one) will be removed.
+    "#);
+}
+
 #[test]
 fn test_describe_author() {
     let mut test_env = TestEnvironment::default();