git: Fix buffer diff crash that could occur during the stage all operation (#47265)

Closes https://github.com/zed-industries/zed/issues/46519

This crash happened because we were incorrectly merging pending and
unstaged hunks together by always using the pending hunk end offset as
the merged hunk bound instead of the max of both hunks.

The fix is to use `max()` when updating `buffer_offset_range.end` during
pending hunk merging, matching the behavior used for unstaged hunk
merging.

Release Notes:

- N/A

crates/buffer_diff/src/buffer_diff.rs

@@ -536,7 +536,9 @@ impl BufferDiffInner<Entity<language::Buffer>> {
                     let next_pending_hunk_offset_range =
                         next_pending_hunk.buffer_range.to_offset(buffer);
                     if next_pending_hunk_offset_range.start <= buffer_offset_range.end {
-                        buffer_offset_range.end = next_pending_hunk_offset_range.end;
+                        buffer_offset_range.end = buffer_offset_range
+                            .end
+                            .max(next_pending_hunk_offset_range.end);
                         pending_hunks_iter.next();
                         continue;
                     }
@@ -2140,6 +2142,72 @@ mod tests {
         }
     }
 
+    #[gpui::test]
+    async fn test_stage_all_with_nested_hunks(cx: &mut TestAppContext) {
+        // This test reproduces a crash where staging all hunks would cause an underflow
+        // when there's one large unstaged hunk containing multiple uncommitted hunks.
+        let head_text = "
+            aaa
+            bbb
+            ccc
+            ddd
+            eee
+            fff
+            ggg
+            hhh
+            iii
+            jjj
+            kkk
+            lll
+        "
+        .unindent();
+
+        let index_text = "
+            aaa
+            bbb
+            CCC-index
+            DDD-index
+            EEE-index
+            FFF-index
+            GGG-index
+            HHH-index
+            III-index
+            JJJ-index
+            kkk
+            lll
+        "
+        .unindent();
+
+        let buffer_text = "
+            aaa
+            bbb
+            ccc-modified
+            ddd
+            eee-modified
+            fff
+            ggg
+            hhh-modified
+            iii
+            jjj
+            kkk
+            lll
+        "
+        .unindent();
+
+        let buffer = Buffer::new(ReplicaId::LOCAL, BufferId::new(1).unwrap(), buffer_text);
+
+        let unstaged_diff = cx.new(|cx| BufferDiff::new_with_base_text(&index_text, &buffer, cx));
+        let uncommitted_diff = cx.new(|cx| {
+            let mut diff = BufferDiff::new_with_base_text(&head_text, &buffer, cx);
+            diff.set_secondary_diff(unstaged_diff);
+            diff
+        });
+
+        uncommitted_diff.update(cx, |diff, cx| {
+            diff.stage_or_unstage_all_hunks(true, &buffer, true, cx);
+        });
+    }
+
     #[gpui::test]
     async fn test_toggling_stage_and_unstage_same_hunk(cx: &mut TestAppContext) {
         let head_text = "