cli: allow overriding ui.editor config by $JJ_EDITOR variable

CHANGELOG.md

@@ -20,7 +20,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   does, all files in the directory will be read, in alphabetical order.
 
 * You can now override the `$EDITOR` environment variable by setting the
-  `ui.editor` config.
+  `ui.editor` config. There is also a new `$JJ_EDITOR` environment variable,
+  which has even higher priority than the config.
 
 ### Fixed bugs
 

src/main.rs

@@ -52,6 +52,9 @@ fn env_overrides() -> config::Config {
     if let Ok(value) = env::var("JJ_TIMESTAMP") {
         builder = builder.set_override("user.timestamp", value).unwrap();
     }
+    if let Ok(value) = env::var("JJ_EDITOR") {
+        builder = builder.set_override("ui.editor", value).unwrap();
+    }
     builder.build().unwrap()
 }
 

tests/test_describe_command.rs

@@ -77,4 +77,12 @@ fn test_describe() {
         .assert()
         .failure();
     assert!(get_stderr_string(&assert).contains("bad-editor-from-config"));
+
+    // `$JJ_EDITOR` overrides `ui.editor` config
+    let assert = test_env
+        .jj_cmd(&repo_path, &["describe"])
+        .env("JJ_EDITOR", "bad-jj-editor-from-env")
+        .assert()
+        .failure();
+    assert!(get_stderr_string(&assert).contains("bad-jj-editor-from-env"));
 }