Expand symlinks before computing relative path for display paths

Fixes #447

CHANGELOG.md

@@ -11,6 +11,11 @@ support them.
 `--list-languages` now shows filenames associated with languages
 (e.g. `Cargo.lock` is TOML) in addition to extensions.
 
+### Display
+
+Symlinks are now expanded before calculating relative paths, resulting
+in relative paths being shown in more cases.
+
 ### Build
 
 Difftastic is now built with Ubuntu 20.04 on GitHub, so prebuilt

src/options.rs

@@ -242,8 +242,15 @@ pub enum FileArgument {
     DevNull,
 }
 
+fn try_canonicalize(path: &Path) -> PathBuf {
+    path.canonicalize().unwrap_or_else(|_| path.into())
+}
+
 fn relative_to_current(path: &Path) -> PathBuf {
     if let Ok(current_path) = std::env::current_dir() {
+        let path = try_canonicalize(path);
+        let current_path = try_canonicalize(&current_path);
+
         if let Ok(rel_path) = path.strip_prefix(current_path) {
             return rel_path.into();
         }