describe: ignore everything below ignore-rest line

This implements "scissor" lines. For example:

    this text is included in the commit message
    JJ: ignore-rest
    this text is not, and is encouraged to be rendered as a diff
    JJ: ignore-rest
    this text is *still not* included in the commit message

When editing multiple commit messages, the `JJ: describe {}` lines
are parsed before the description is cleaned up. That means that the
following will correctly add descriptions to multiple commits:

    JJ: describe aaaaaaaaaaaa
    this text is included in the first commit message
    JJ: ignore-rest
    scissored...
    
    JJ: describe bbbbbbbbbbbb
    this text is included in the first commit message
    JJ: ignore-rest
    scissored...

CHANGELOG.md

@@ -78,6 +78,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * New `$marker_length` variable to allow merge tools to support longer conflict
   markers (equivalent to "%L" for Git merge drivers).
 
+* `jj describe` now accepts a `JJ: ignore-rest` line that ignores everything
+  below it, similar to a "scissor line" in git. When editing multiple commits,
+  only ignore until the next `JJ: describe` line.
+
 ### Fixed bugs
 
 * The `$NO_COLOR` environment variable must now be non-empty to be respected.

cli/src/config/templates.toml

@@ -29,8 +29,6 @@ if(overridden,
 ) ++ "\n"
 '''
 
-# TODO: Provide hook point for diff customization (#1946)? We might want a
-# syntax to comment out full text diffs without using the "JJ:" prefix.
 draft_commit_description = '''
 concat(
   description,

cli/src/description_util.rs

@@ -5,6 +5,7 @@ use std::path::Path;
 use bstr::ByteVec as _;
 use indexmap::IndexMap;
 use indoc::indoc;
+use itertools::FoldWhile;
 use itertools::Itertools;
 use jj_lib::backend::CommitId;
 use jj_lib::commit::Commit;
@@ -28,8 +29,17 @@ where
 {
     let description = lines
         .into_iter()
-        .filter(|line| !line.as_ref().starts_with("JJ:"))
-        .fold(String::new(), |acc, line| acc + line.as_ref() + "\n");
+        .fold_while(String::new(), |acc, line| {
+            let line = line.as_ref();
+            if line.strip_prefix("JJ: ignore-rest").is_some() {
+                FoldWhile::Done(acc)
+            } else if line.starts_with("JJ:") {
+                FoldWhile::Continue(acc)
+            } else {
+                FoldWhile::Continue(acc + line + "\n")
+            }
+        })
+        .into_inner();
     text_util::complete_newline(description.trim_matches('\n'))
 }
 

cli/tests/test_describe_command.rs

@@ -140,6 +140,35 @@ fn test_describe() {
     std::fs::write(&edit_script, "fail").unwrap();
     let stderr = test_env.jj_cmd_failure(&repo_path, &["describe"]);
     assert!(stderr.contains("exited with an error"));
+
+    // ignore everything after the first ignore-rest line
+    std::fs::write(
+        &edit_script,
+        indoc! {"
+            write
+            description from editor
+
+            content of message from editor
+            JJ: ignore-rest
+            content after ignore line should not be included
+            JJ: ignore-rest
+            ignore everything until EOF or next description
+        "},
+    )
+    .unwrap();
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["describe"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r#"
+    Working copy now at: qpvuntsm 10fa2dc7 (empty) description from editor
+    Parent commit      : zzzzzzzz 00000000 (empty) (no description set)
+    "#);
+    let stdout =
+        test_env.jj_cmd_success(&repo_path, &["log", "--no-graph", "-r@", "-Tdescription"]);
+    insta::assert_snapshot!(stdout, @r#"
+    description from editor
+
+    content of message from editor
+    "#);
 }
 
 #[test]
@@ -384,6 +413,42 @@ fn test_describe_multiple_commits() {
     std::fs::write(&edit_script, "fail").unwrap();
     let stderr = test_env.jj_cmd_failure(&repo_path, &["describe", "@", "@-"]);
     assert!(stderr.contains("exited with an error"));
+
+    // describe lines should take priority over ignore-rest
+    std::fs::write(
+        &edit_script,
+        indoc! {"
+            write
+            JJ: describe 0d76a92ca7cc -------
+            description from editor for @-
+
+            JJ: ignore-rest
+            content after ignore-rest should not be included
+
+            JJ: describe a42f5755e688 -------
+            description from editor for @--
+
+            JJ: ignore-rest
+            each commit should skip their own ignore-rest
+        "},
+    )
+    .unwrap();
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["describe", "@-", "@--"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r#"
+    Updated 2 commits
+    Rebased 1 descendant commits
+    Working copy now at: kkmpptxz 1d7701ee (empty) description from editor of @
+    Parent commit      : rlvkpnrz 5389926e (empty) description from editor for @-
+    "#);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r#"
+    @  1d7701eec9bc description from editor of @
+    │
+    │  further commit message of @
+    ○  5389926ebed6 description from editor for @-
+    ○  eaa8547ae37a description from editor for @--
+    ◆  000000000000
+    "#);
 }
 
 #[test]

docs/config.md

@@ -185,6 +185,8 @@ concat(
     "\nJJ: This commit contains the following changes:\n", "",
     indent("JJ:     ", diff.stat(72)),
   ),
+  "\nJJ: ignore-rest\n",
+  diff.git(),
 )
 '''
 ```