Make relativize_path_maybe directly update the path argument

This simplifies the call sites.
Also skip updating if the path is already absolute, and handle
'/dev/null' on Windows so it is no longer converted to '\dev\null'.

src/handlers/diff_header.rs

@@ -84,12 +84,11 @@ impl<'a> StateMachine<'a> {
             return Ok(false);
         }
 
-        let (path_or_mode, file_event) =
+        let (mut path_or_mode, file_event) =
             parse_diff_header_line(&self.line, self.source == Source::GitDiff);
 
-        self.minus_file = utils::path::relativize_path_maybe(&path_or_mode, self.config)
-            .map(|p| p.to_string_lossy().into_owned())
-            .unwrap_or(path_or_mode);
+        utils::path::relativize_path_maybe(&mut path_or_mode, self.config);
+        self.minus_file = path_or_mode;
         self.minus_file_event = file_event;
 
         if self.source == Source::DiffUnified {
@@ -121,12 +120,11 @@ impl<'a> StateMachine<'a> {
             return Ok(false);
         }
         let mut handled_line = false;
-        let (path_or_mode, file_event) =
+        let (mut path_or_mode, file_event) =
             parse_diff_header_line(&self.line, self.source == Source::GitDiff);
 
-        self.plus_file = utils::path::relativize_path_maybe(&path_or_mode, self.config)
-            .map(|p| p.to_string_lossy().into_owned())
-            .unwrap_or(path_or_mode);
+        utils::path::relativize_path_maybe(&mut path_or_mode, self.config);
+        self.plus_file = path_or_mode;
         self.plus_file_event = file_event;
         self.painter
             .set_syntax(get_filename_from_diff_header_line_file_path(

src/utils/path.rs

@@ -25,16 +25,24 @@ pub fn absolute_path(relative_path: &str, config: &Config) -> Option<PathBuf> {
     .map(normalize_path)
 }
 
-/// Relativize path if delta config demands that and paths are not already relativized by git.
-pub fn relativize_path_maybe(path: &str, config: &Config) -> Option<PathBuf> {
-    if config.relative_paths && !calling_process().paths_in_input_are_relative_to_cwd() {
-        if let Some(base) = config.cwd_relative_to_repo_root.as_deref() {
-            pathdiff::diff_paths(path, base)
-        } else {
-            None
+/// Relativize `path` if delta `config` demands that and paths are not already relativized by git.
+pub fn relativize_path_maybe(path: &mut String, config: &Config) {
+    let mut inner_relativize = || -> Option<()> {
+        let base = config.cwd_relative_to_repo_root.as_deref()?;
+        let relative_path = pathdiff::diff_paths(&path, base)?;
+        if relative_path.is_relative() {
+            #[cfg(target_os = "windows")]
+            // '/dev/null' is converted to '\dev\null' and considered relative. Work
+            // around that by leaving all paths like that untouched:
+            if relative_path.starts_with(Path::new(r"\")) {
+                return None;
+            }
+            *path = relative_path.to_string_lossy().into_owned();
         }
-    } else {
-        None
+        Some(())
+    };
+    if config.relative_paths && !calling_process().paths_in_input_are_relative_to_cwd() {
+        let _ = inner_relativize();
     }
 }