cli: rename diff.format to ui.diff.format

I felt that the config is too narrow to have it's own top-level [diff]
section, and I couldn't think of another good place to have it. I'm
happy to hear other suggestions.

CHANGELOG.md

@@ -41,6 +41,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * The global `--no-commit-working-copy` is now called `--ignore-working-copy`.
 
+* The `diff.format` config option is now called `ui.diff.format`. The old name
+  is still supported for now.
+
 ### New features
 
 * The default log format now uses the committer timestamp instead of the author

src/diff_util.rs

@@ -90,7 +90,13 @@ fn diff_formats_from_args(args: &DiffFormatArgs) -> Vec<DiffFormat> {
 }
 
 fn default_diff_format(settings: &UserSettings) -> DiffFormat {
-    match settings.config().get_string("diff.format").as_deref() {
+    match settings
+        .config()
+        .get_string("ui.diff.format")
+        // old config name
+        .or_else(|_| settings.config().get_string("diff.format"))
+        .as_deref()
+    {
         Ok("summary") => DiffFormat::Summary,
         Ok("git") => DiffFormat::Git,
         Ok("color-words") => DiffFormat::ColorWords,

tests/test_log_command.rs

@@ -114,7 +114,7 @@ fn test_log_with_or_without_diff() {
             "description",
             "-p",
             "-s",
-            "--config-toml=diff.format='summary'",
+            "--config-toml=ui.diff.format='summary'",
         ],
     );
     insta::assert_snapshot!(stdout, @r###"