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/agent_ui/src/acp/message_editor.rs

@@ -1423,7 +1423,7 @@ mod tests {
             rel_path("b/eight.txt"),
         ];
 
-        let slash = PathStyle::local().separator();
+        let slash = PathStyle::local().primary_separator();
 
         let mut opened_editors = Vec::new();
         for path in paths {

crates/agent_ui/src/acp/thread_view.rs

@@ -3989,7 +3989,7 @@ impl AcpThreadView {
                 let file = buffer.read(cx).file()?;
                 let path = file.path();
                 let path_style = file.path_style(cx);
-                let separator = file.path_style(cx).separator();
+                let separator = file.path_style(cx).primary_separator();
 
                 let file_path = path.parent().and_then(|parent| {
                     if parent.is_empty() {