multi_buffer: Remove redundant buffer id field (#43459)

It is easy for us to get the two fields out of sync causing weird
problems, there is no reason to have both here so.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Co-authored by: Antonio Scandurra <antonio@zed.dev>

crates/git_ui/src/project_diff.rs

@@ -383,12 +383,8 @@ impl ProjectDiff {
             .collect::<Vec<_>>();
         if !ranges.iter().any(|range| range.start != range.end) {
             selection = false;
-            if let Some((excerpt_id, buffer, range)) = self.editor.read(cx).active_excerpt(cx) {
-                ranges = vec![multi_buffer::Anchor::range_in_buffer(
-                    excerpt_id,
-                    buffer.read(cx).remote_id(),
-                    range,
-                )];
+            if let Some((excerpt_id, _, range)) = self.editor.read(cx).active_excerpt(cx) {
+                ranges = vec![multi_buffer::Anchor::range_in_buffer(excerpt_id, range)];
             } else {
                 ranges = Vec::default();
             }
@@ -488,7 +484,11 @@ impl ProjectDiff {
         let snapshot = buffer.read(cx).snapshot();
         let diff_read = diff.read(cx);
         let diff_hunk_ranges = diff_read
-            .hunks_intersecting_range(Anchor::MIN..Anchor::MAX, &snapshot, cx)
+            .hunks_intersecting_range(
+                Anchor::min_max_range_for_buffer(diff_read.buffer_id),
+                &snapshot,
+                cx,
+            )
             .map(|diff_hunk| diff_hunk.buffer_range);
         let conflicts = conflict_addon
             .conflict_set(snapshot.remote_id())