config: also check XDG_CONFIG_HOME on MacOS

This replaces the `dirs` crate with the more lightweight and flexible
`etcetera` crate, which also lets us use XDG config on MacOS. Will search in
$HOME, the platform-specific dir, and $XDG_CONFIG_HOME (if different from the
platform-specific dir, like on MacOS).

I changed some tests to more accurately describe the desired behavior and added
a couple of new ones.

Fixes #3434

CHANGELOG.md

@@ -15,6 +15,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * The default template alias `builtin_log_root(change_id: ChangeId, commit_id: CommitId)` was replaced by `format_root_commit(root: Commit)`.
 
+* On MacOS, jj will now also look for its config in `$XDG_CONFIG_HOME` in addition to `~/Library/Application Support/jj/` and `$HOME/.jjconfig.toml`
+
 ### New features
 
 * The list of conflicted paths is printed whenever the working copy changes.

Cargo.toml

@@ -43,9 +43,9 @@ config = { version = "0.13.4", default-features = false, features = ["toml"] }
 criterion = "0.5.1"
 crossterm = { version = "0.27", default-features = false }
 digest = "0.10.7"
-dirs = "5.0.1"
 either = "1.10.0"
 esl01-renderdag = "0.3.0"
+etcetera = "0.8.0"
 futures = "0.3.30"
 git2 = "0.18.3"
 gix = { version = "0.61.0", default-features = false, features = [

cli/Cargo.toml

@@ -43,8 +43,8 @@ clap_mangen = { workspace = true }
 config = { workspace = true }
 criterion = { workspace = true, optional = true }
 crossterm = { workspace = true }
-dirs = { workspace = true }
 esl01-renderdag = { workspace = true }
+etcetera = { workspace = true }
 futures = { workspace = true }
 git2 = { workspace = true }
 gix = { workspace = true }

cli/src/cli_util.rs

@@ -83,7 +83,7 @@ use crate::command_error::{
 };
 use crate::commit_templater::{CommitTemplateLanguage, CommitTemplateLanguageExtension};
 use crate::config::{
-    new_config_path, AnnotatedValue, CommandNameAndArgs, ConfigSource, LayeredConfigs,
+    new_or_existing_config_path, AnnotatedValue, CommandNameAndArgs, ConfigSource, LayeredConfigs,
 };
 use crate::formatter::{FormatRecorder, Formatter, PlainTextFormatter};
 use crate::git_util::{
@@ -1984,9 +1984,8 @@ pub fn get_new_config_file_path(
 ) -> Result<PathBuf, CommandError> {
     let edit_path = match config_source {
         // TODO(#531): Special-case for editors that can't handle viewing directories?
-        ConfigSource::User => {
-            new_config_path()?.ok_or_else(|| user_error("No repo config path found to edit"))?
-        }
+        ConfigSource::User => new_or_existing_config_path()?
+            .ok_or_else(|| user_error("No repo config path found to edit"))?,
         ConfigSource::Repo => command.workspace_loader()?.repo_path().join("config.toml"),
         _ => {
             return Err(user_error(format!(