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/fuzzy/src/paths.rs

@@ -107,7 +107,7 @@ pub fn match_fixed_path_set(
                 .display(path_style)
                 .chars()
                 .collect::<Vec<_>>();
-            path_prefix_chars.extend(path_style.separator().chars());
+            path_prefix_chars.extend(path_style.primary_separator().chars());
             let lowercase_pfx = path_prefix_chars
                 .iter()
                 .map(|c| c.to_ascii_lowercase())