Clear pending staged/unstaged diff hunks hunks when writing to the git index fails (#26173)

Release Notes:

- Git Beta: Fixed a bug where discarding a hunk in the project diff view
performed two concurrent saves of the buffer.
- Git Beta: Fixed an issue where diff hunks appeared in the wrong state
after failing to write to the git index.

crates/multi_buffer/src/multi_buffer.rs

@@ -250,6 +250,7 @@ impl DiffState {
                     }
                 }
                 BufferDiffEvent::LanguageChanged => this.buffer_diff_language_changed(diff, cx),
+                _ => {}
             }),
             diff,
         }