Highlight merge conflicts and provide for resolving them (#28065)

TODO:

- [x] Make it work in the project diff:
  - [x] Support non-singleton buffers
  - [x] Adjust excerpt boundaries to show full conflicts
- [x] Write tests for conflict-related events and state management
- [x] Prevent hunk buttons from appearing inside conflicts
- [x] Make sure it works over SSH, collab
- [x] Allow separate theming of markers

Bonus:

- [ ] Count of conflicts in toolbar
- [ ] Keyboard-driven navigation and resolution
- [ ] ~~Inlay hints to contextualize "ours"/"theirs"~~

Release Notes:

- Implemented initial support for resolving merge conflicts.

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/git_ui/src/git_panel.rs

@@ -447,7 +447,7 @@ impl GitPanel {
                         .ok();
                 }
                 GitStoreEvent::RepositoryUpdated(_, _, _) => {}
-                GitStoreEvent::JobsUpdated => {}
+                GitStoreEvent::JobsUpdated | GitStoreEvent::ConflictsUpdated => {}
             },
         )
         .detach();
@@ -1650,7 +1650,7 @@ impl GitPanel {
         if let Some(merge_message) = self
             .active_repository
             .as_ref()
-            .and_then(|repo| repo.read(cx).merge_message.as_ref())
+            .and_then(|repo| repo.read(cx).merge.message.as_ref())
         {
             return Some(merge_message.to_string());
         }