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/editor/src/element.rs

@@ -1,6 +1,7 @@
 use crate::{
     ActiveDiagnostic, BlockId, COLUMNAR_SELECTION_MODIFIERS, CURSORS_VISIBLE_FOR,
-    ChunkRendererContext, ChunkReplacement, ContextMenuPlacement, CursorShape, CustomBlockId,
+    ChunkRendererContext, ChunkReplacement, ConflictsOurs, ConflictsOursMarker, ConflictsOuter,
+    ConflictsTheirs, ConflictsTheirsMarker, ContextMenuPlacement, CursorShape, CustomBlockId,
     DisplayDiffHunk, DisplayPoint, DisplayRow, DocumentHighlightRead, DocumentHighlightWrite,
     EditDisplayMode, Editor, EditorMode, EditorSettings, EditorSnapshot, EditorStyle,
     FILE_HEADER_HEIGHT, FocusedBlock, GutterDimensions, HalfPageDown, HalfPageUp, HandleInput,
@@ -4036,6 +4037,7 @@ impl EditorElement {
         line_height: Pixels,
         scroll_pixel_position: gpui::Point<Pixels>,
         display_hunks: &[(DisplayDiffHunk, Option<Hitbox>)],
+        highlighted_rows: &BTreeMap<DisplayRow, LineHighlight>,
         editor: Entity<Editor>,
         window: &mut Window,
         cx: &mut App,
@@ -4064,6 +4066,22 @@ impl EditorElement {
                 {
                     continue;
                 }
+                if highlighted_rows
+                    .get(&display_row_range.start)
+                    .and_then(|highlight| highlight.type_id)
+                    .is_some_and(|type_id| {
+                        [
+                            TypeId::of::<ConflictsOuter>(),
+                            TypeId::of::<ConflictsOursMarker>(),
+                            TypeId::of::<ConflictsOurs>(),
+                            TypeId::of::<ConflictsTheirs>(),
+                            TypeId::of::<ConflictsTheirsMarker>(),
+                        ]
+                        .contains(&type_id)
+                    })
+                {
+                    continue;
+                }
                 let row_ix = (display_row_range.start - row_range.start).0 as usize;
                 if row_infos[row_ix].diff_status.is_none() {
                     continue;
@@ -4258,14 +4276,21 @@ impl EditorElement {
                                            highlight_row_end: DisplayRow,
                                            highlight: crate::LineHighlight,
                                            edges| {
+                    let mut origin_x = layout.hitbox.left();
+                    let mut width = layout.hitbox.size.width;
+                    if !highlight.include_gutter {
+                        origin_x += layout.gutter_hitbox.size.width;
+                        width -= layout.gutter_hitbox.size.width;
+                    }
+
                     let origin = point(
-                        layout.hitbox.origin.x,
+                        origin_x,
                         layout.hitbox.origin.y
                             + (highlight_row_start.as_f32() - scroll_top)
                                 * layout.position_map.line_height,
                     );
                     let size = size(
-                        layout.hitbox.size.width,
+                        width,
                         layout.position_map.line_height
                             * highlight_row_end.next_row().minus(highlight_row_start) as f32,
                     );
@@ -6789,10 +6814,16 @@ impl Element for EditorElement {
                             } else {
                                 background_color.opacity(0.36)
                             }),
+                            include_gutter: true,
+                            type_id: None,
                         };
 
-                        let filled_highlight =
-                            solid_background(background_color.opacity(hunk_opacity)).into();
+                        let filled_highlight = LineHighlight {
+                            background: solid_background(background_color.opacity(hunk_opacity)),
+                            border: None,
+                            include_gutter: true,
+                            type_id: None,
+                        };
 
                         let background = if Self::diff_hunk_hollow(diff_status, cx) {
                             hollow_highlight
@@ -7551,6 +7582,7 @@ impl Element for EditorElement {
                             line_height,
                             scroll_pixel_position,
                             &display_hunks,
+                            &highlighted_rows,
                             self.editor.clone(),
                             window,
                             cx,