Implement --diff-args

src/cli.rs

@@ -317,6 +317,16 @@ pub struct Opt {
     #[arg(long = "detect-dark-light", value_enum, default_value_t = DetectDarkLight::default())]
     pub detect_dark_light: DetectDarkLight,
 
+    #[arg(long = "diff-args", default_value = "", value_name = "STRING")]
+    /// Arguments to pass to `git diff` when using delta to diff two files.
+    ///
+    /// E.g. `delta --diff-args=-U999 file_1 file_2` is equivalent to
+    /// `git diff -U999 --no-index --color file_1 file_2 | delta`.
+    ///
+    /// However, if you use process substitution instead of real file paths, it falls back to `diff -u` instead of `git
+    /// diff`.
+    pub diff_args: String,
+
     #[arg(long = "diff-highlight")]
     /// Emulate diff-highlight.
     ///
@@ -1101,12 +1111,10 @@ pub struct Opt {
     /// Deprecated: use --true-color.
     pub _24_bit_color: Option<String>,
 
-    /// First file to be compared when delta is being used in diff mode
-    ///
-    /// `delta file_1 file_2` is equivalent to `diff -u file_1 file_2 | delta`.
+    /// First file to be compared when delta is being used to diff two files.
     pub minus_file: Option<PathBuf>,
 
-    /// Second file to be compared when delta is being used in diff mode.
+    /// Second file to be compared when delta is being used to diff two files.
     pub plus_file: Option<PathBuf>,
 
     #[arg(skip)]

src/config.rs

@@ -53,6 +53,7 @@ pub struct Config {
     pub cwd_relative_to_repo_root: Option<String>,
     pub decorations_width: cli::Width,
     pub default_language: String,
+    pub diff_args: String,
     pub diff_stat_align_width: usize,
     pub error_exit_code: i32,
     pub file_added_label: String,
@@ -294,6 +295,7 @@ impl From<cli::Opt> for Config {
             cwd_relative_to_repo_root,
             decorations_width: opt.computed.decorations_width,
             default_language: opt.default_language,
+            diff_args: opt.diff_args,
             diff_stat_align_width: opt.diff_stat_align_width,
             error_exit_code: 2, // Use 2 for error because diff uses 0 and 1 for non-error.
             file_added_label,

src/options/set.rs

@@ -140,6 +140,7 @@ pub fn set_options(
             commit_regex,
             commit_style,
             default_language,
+            diff_args,
             diff_stat_align_width,
             file_added_label,
             file_copied_label,

src/subcommands/diff.rs

@@ -24,12 +24,14 @@ pub fn diff(
         |f: &Path| f.starts_with("/proc/self/fd/") || f.starts_with("/dev/fd/");
 
     let diff_cmd = if via_process_substitution(minus_file) || via_process_substitution(plus_file) {
-        ["diff", "-u", "--"].as_slice()
+        format!("diff -u {} --", config.diff_args)
     } else {
-        ["git", "diff", "--no-index", "--color", "--"].as_slice()
+        format!("git diff --no-index --color {} --", config.diff_args)
     };
 
-    let diff_bin = diff_cmd[0];
+    let mut diff_cmd = diff_cmd.split_whitespace();
+    let diff_bin = diff_cmd.next().unwrap();
+
     let diff_path = match grep_cli::resolve_binary(PathBuf::from(diff_bin)) {
         Ok(path) => path,
         Err(err) => {
@@ -39,7 +41,7 @@ pub fn diff(
     };
 
     let diff_process = process::Command::new(diff_path)
-        .args(&diff_cmd[1..])
+        .args(diff_cmd)
         .args([minus_file, plus_file])
         .stdout(process::Stdio::piped())
         .spawn();