Fix git features not working when a Windows host collaborates with a unix guest (#43515)

We were using `std::path::Path::strip_prefix` to determine which
repository an absolute path belongs to, which doesn't work when the
paths are Windows-style but the code is running on unix. Replace it with
a platform-agnostic implementation of `strip_prefix`.

Release Notes:

- Fixed git features not working when a Windows host collaborates with a
unix guest

crates/settings_ui/src/settings_ui.rs

@@ -2192,7 +2192,7 @@ impl SettingsWindow {
                         format!(
                             "{}{}{}",
                             directory_name,
-                            path_style.separator(),
+                            path_style.primary_separator(),
                             path.display(path_style)
                         )
                     }