Improve handling of /dev/null paths when there are two CLI argsuments

When git calls us, we always know the file name. If we're called with
two arguments and one is /dev/null, use the other for language
detection and display.

CHANGELOG.md

@@ -13,6 +13,11 @@ TypeScript.
 Fixed an issue with inline mode where there was a blank line between
 every line.
 
+### Command Line Interface
+
+Fixed language detection and filename display when one path was
+`/dev/null` and difftastic was invoked with two arguments.
+
 ## 0.30 (released 4th July 2022)
 
 ### Parsing

src/display/style.rs

@@ -380,19 +380,24 @@ pub fn header(
         display_options.use_color,
         display_options.background_color,
     );
+    let lhs_path_pretty = apply_header_color(
+        lhs_display_path,
+        display_options.use_color,
+        display_options.background_color,
+    );
     if hunk_num == 1 && lhs_display_path != rhs_display_path && display_options.in_vcs {
-        let lhs_path_pretty = apply_header_color(
-            lhs_display_path,
-            display_options.use_color,
-            display_options.background_color,
-        );
         let renamed = format!("Renamed {} to {}", lhs_path_pretty, rhs_path_pretty,);
         format!(
             "{}\n{} --- {}{}",
             renamed, rhs_path_pretty, divider, language_name
         )
     } else {
-        format!("{} --- {}{}", rhs_path_pretty, divider, language_name)
+        let path_pretty = if lhs_display_path == "/dev/null" {
+            rhs_path_pretty
+        } else {
+            lhs_path_pretty
+        };
+        format!("{} --- {}{}", path_pretty, divider, language_name)
     }
 }
 

src/main.rs

@@ -242,16 +242,14 @@ fn diff_file_content(
         rhs_src.pop();
     }
 
-    // TODO: take a Path directly instead.
-    let guess_path = Path::new(&rhs_display_path);
-
     // Take the larger of the two files when guessing the
     // language. This is useful when we've added or removed a whole
     // file.
-    let guess_src = if lhs_src.len() > rhs_src.len() {
+    let (guess_src, guess_path) = if lhs_src.len() > rhs_src.len() {
-        &lhs_src
+        // TODO: take a Path directly instead.
+        (&lhs_src, Path::new(&lhs_display_path))
     } else {
-        &rhs_src
+        (&rhs_src, Path::new(&rhs_display_path))
     };
     let language = language_override.or_else(|| guess(guess_path, guess_src));
     let lang_config = language.map(tsp::from_language);