Fix two bugs in new diff hunk handling (#23990)

Closes https://github.com/zed-industries/zed/issues/23981

Release Notes:

- Fixed a crash that could happen when expanding certain diff hunks
- Fixed a bug where diff hunks were not syntax highlighted when
reopening a project with previously-opened buffers.

crates/multi_buffer/src/multi_buffer.rs

@@ -3484,7 +3484,8 @@ impl MultiBufferSnapshot {
                 let region = cursor.region()?;
                 let buffer_start = if region.is_main_buffer {
                     let start_overshoot = range.start.saturating_sub(region.range.start.key);
-                    region.buffer_range.start.key + start_overshoot
+                    (region.buffer_range.start.key + start_overshoot)
+                        .min(region.buffer_range.end.key)
                 } else {
                     cursor.main_buffer_position()?.key
                 };