git: Improve diff performance (#54435)

Previously, passing an empty range to `range_to_buffer_range` would
return no ranges, which would trigger a fallback case in the git diff
that caused the entire multibuffer to be invalidated every frame.

This PR fixes `range_to_buffer_range`, so that it returns a sensible
range if the input range is empty, preventing the fallback behaviour and
edit expansion.

Self-Review Checklist:

- [ ] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

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

---------

Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>

crates/multi_buffer/src/multi_buffer.rs

@@ -3636,6 +3636,7 @@ impl MultiBufferSnapshot {
         result
     }
 
+    /// Callers should not provide a range where `end < start`
     pub fn range_to_buffer_ranges<T: ToOffset>(
         &self,
         range: Range<T>,
@@ -3647,6 +3648,7 @@ impl MultiBufferSnapshot {
         let mut cursor = self.cursor::<MultiBufferOffset, BufferOffset>();
         let start = range.start.to_offset(self);
         let end = range.end.to_offset(self);
+        let range_non_empty = end > start;
         cursor.seek(&start);
 
         let mut result: Vec<(
@@ -3655,7 +3657,7 @@ impl MultiBufferSnapshot {
             ExcerptRange<text::Anchor>,
         )> = Vec::new();
         while let Some(region) = cursor.region() {
-            if region.range.start >= end {
+            if region.range.start > end || (region.range.start == end && range_non_empty) {
                 break;
             }
             if region.is_main_buffer {