Persist the default Git diff layout

src/config.rs

@@ -46,6 +46,25 @@ pub const GIT_DIFF_ALGORITHMS: [GitDiffAlgorithm; 3] = [
     GitDiffAlgorithm::Minimal,
 ];
 
+pub const GIT_DIFF_LAYOUTS: [GitDiffLayout; 2] = [GitDiffLayout::Unified, GitDiffLayout::Split];
+
+#[derive(Clone, Copy, Debug, Default, Deserialize, Eq, PartialEq, Serialize)]
+#[serde(rename_all = "kebab-case")]
+pub enum GitDiffLayout {
+    #[default]
+    Unified,
+    Split,
+}
+
+impl fmt::Display for GitDiffLayout {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        formatter.write_str(match self {
+            Self::Unified => "1 column",
+            Self::Split => "2 columns",
+        })
+    }
+}
+
 #[derive(Clone, Copy, Debug, Default, Deserialize, Eq, PartialEq, Serialize)]
 #[serde(rename_all = "kebab-case")]
 pub enum GitDiffAlgorithm {
@@ -96,6 +115,7 @@ impl fmt::Display for GitDiffWhitespace {
 #[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
 #[serde(default, deny_unknown_fields)]
 pub struct GitConfig {
+    pub diff_layout: GitDiffLayout,
     pub diff_algorithm: GitDiffAlgorithm,
     pub context_lines: u32,
     pub interhunk_lines: u32,
@@ -107,6 +127,7 @@ pub struct GitConfig {
 impl Default for GitConfig {
     fn default() -> Self {
         Self {
+            diff_layout: GitDiffLayout::Unified,
             diff_algorithm: GitDiffAlgorithm::Default,
             context_lines: 3,
             interhunk_lines: 0,
@@ -282,6 +303,7 @@ mod tests {
                 ..RuntimePreferences::default()
             },
             git: GitConfig {
+                diff_layout: GitDiffLayout::Split,
                 diff_algorithm: GitDiffAlgorithm::Patience,
                 context_lines: 5,
                 whitespace: GitDiffWhitespace::IgnoreEndOfLine,
@@ -297,7 +319,7 @@ mod tests {
             "model: glm-5.2\na2ui_enabled: false\n\
              generation:\n  context_tokens: 65536\n  reasoning_mode: none\n\
              runtime:\n  ssd:\n    enabled: true\n    cache: 64GB\n\
-             git:\n  diff_algorithm: patience\n  context_lines: 5\n  whitespace: ignore-end-of-line\n"
+             git:\n  diff_layout: split\n  diff_algorithm: patience\n  context_lines: 5\n  whitespace: ignore-end-of-line\n"
         );
         assert_eq!(Config::load(&path).unwrap(), config);
         fs::remove_dir_all(&directory).unwrap();