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

@@ -73,6 +73,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   conflicts to be materialized and parsed correctly in files which already
   contain lines that look like conflict markers.
 
+* `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/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 == "JJ: ignore-rest" {
+                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'))
 }
 

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(),
 )
 '''
 ```