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/project/src/git_store.rs

@@ -3222,10 +3222,8 @@ impl RepositorySnapshot {
         abs_path: &Path,
         path_style: PathStyle,
     ) -> Option<RepoPath> {
-        abs_path
-            .strip_prefix(&work_directory_abs_path)
-            .ok()
-            .and_then(|path| RepoPath::from_std_path(path, path_style).ok())
+        let rel_path = path_style.strip_prefix(abs_path, work_directory_abs_path)?;
+        Some(RepoPath::from_rel_path(&rel_path))
     }
 
     pub fn had_conflict_on_last_merge_head_change(&self, repo_path: &RepoPath) -> bool {