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/toolchain_selector/src/toolchain_selector.rs

@@ -876,7 +876,7 @@ impl ToolchainSelectorDelegate {
             .strip_prefix(&worktree_root)
             .ok()
             .and_then(|suffix| suffix.to_str())
-            .map(|suffix| format!(".{}{suffix}", path_style.separator()).into())
+            .map(|suffix| format!(".{}{suffix}", path_style.primary_separator()).into())
             .unwrap_or(path)
     }
 }