feat(diff): add ui.diff-instructions option to suppress JJ-INSTRUCTIONS file

CHANGELOG.md

@@ -291,6 +291,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   configured by `template-aliases.<name> = <expression>`. Be aware that
   the template syntax isn't documented yet and is likely to change.
 
+* The `ui.diff-instructions` config setting can be set to `false` to inhibit the
+  creation of the `JJ-INSTRUCTIONS` file as part of diff editing.
+
 ### Fixed bugs
 
 * When sharing the working copy with a Git repo, we used to forget to export

docs/config.md

@@ -327,6 +327,13 @@ merge-tools.kdiff3.edit-args = [
     "--merge", "--cs", "CreateBakFiles=0", "$left", "$right"]
 ```
 
+### `JJ-INSTRUCTIONS`
+
+When editing a diff, jj will include a synthetic file called `JJ-INSTRUCTIONS`
+in the diff with instructions on how to edit the diff. Any changes you make to
+this file will be ignored. To suppress the creation of this file, set
+`ui.diff-instructions = false`.
+
 ### Using Vim as a diff editor
 
 Using `ui.diff-editor = "vimdiff"` is possible but not recommended. For a better

lib/src/settings.rs

@@ -162,6 +162,10 @@ impl UserSettings {
             .unwrap_or(false)
     }
 
+    pub fn diff_instructions(&self) -> bool {
+        self.config.get_bool("ui.diff-instructions").unwrap_or(true)
+    }
+
     pub fn config(&self) -> &config::Config {
         &self.config
     }

src/merge_tools.rs

@@ -345,7 +345,8 @@ pub fn edit_diff(
     let instructions_path = right_wc_dir.join("JJ-INSTRUCTIONS");
     // In the unlikely event that the file already exists, then the user will simply
     // not get any instructions.
-    let add_instructions = !instructions.is_empty() && !instructions_path.exists();
+    let add_instructions =
+        settings.diff_instructions() && !instructions.is_empty() && !instructions_path.exists();
     if add_instructions {
         // TODO: This can be replaced with std::fs::write. Is this used in other places
         // as well?