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/file_finder/src/file_finder.rs

@@ -1060,7 +1060,7 @@ impl FileFinderDelegate {
                         (
                             filename.to_string(),
                             Vec::new(),
-                            prefix.display(path_style).to_string() + path_style.separator(),
+                            prefix.display(path_style).to_string() + path_style.primary_separator(),
                             Vec::new(),
                         )
                     } else {
@@ -1071,7 +1071,7 @@ impl FileFinderDelegate {
                                 .map_or(String::new(), |f| f.to_string_lossy().into_owned()),
                             Vec::new(),
                             entry_path.absolute.parent().map_or(String::new(), |path| {
-                                path.to_string_lossy().into_owned() + path_style.separator()
+                                path.to_string_lossy().into_owned() + path_style.primary_separator()
                             }),
                             Vec::new(),
                         )

crates/file_finder/src/file_finder_tests.rs

@@ -1598,7 +1598,7 @@ async fn test_history_match_positions(cx: &mut gpui::TestAppContext) {
         assert_eq!(file_label.highlight_indices(), &[0, 1, 2]);
         assert_eq!(
             path_label.text(),
-            format!("test{}", PathStyle::local().separator())
+            format!("test{}", PathStyle::local().primary_separator())
         );
         assert_eq!(path_label.highlight_indices(), &[] as &[usize]);
     });

crates/file_finder/src/open_path_prompt.rs

@@ -559,7 +559,7 @@ impl PickerDelegate for OpenPathDelegate {
                         parent_path,
                         candidate.path.string,
                         if candidate.is_dir {
-                            path_style.separator()
+                            path_style.primary_separator()
                         } else {
                             ""
                         }
@@ -569,7 +569,7 @@ impl PickerDelegate for OpenPathDelegate {
                         parent_path,
                         candidate.path.string,
                         if candidate.is_dir {
-                            path_style.separator()
+                            path_style.primary_separator()
                         } else {
                             ""
                         }
@@ -826,7 +826,13 @@ impl PickerDelegate for OpenPathDelegate {
     }
 
     fn placeholder_text(&self, _window: &mut Window, _cx: &mut App) -> Arc<str> {
-        Arc::from(format!("[directory{}]filename.ext", self.path_style.separator()).as_str())
+        Arc::from(
+            format!(
+                "[directory{}]filename.ext",
+                self.path_style.primary_separator()
+            )
+            .as_str(),
+        )
     }
 
     fn separators_after_indices(&self) -> Vec<usize> {