Flush minus/plus line buffers in additional code locations

Fixes #1002

src/handlers/diff_header.rs

@@ -83,6 +83,7 @@ impl<'a> StateMachine<'a> {
                 ));
         }
 
+        self.painter.paint_buffered_minus_and_plus_lines();
         // In color_only mode, raw_line's structure shouldn't be changed.
         // So it needs to avoid fn _handle_diff_header_header_line
         // (it connects the plus_file and minus_file),
@@ -127,6 +128,7 @@ impl<'a> StateMachine<'a> {
             ));
         self.current_file_pair = Some((self.minus_file.clone(), self.plus_file.clone()));
 
+        self.painter.paint_buffered_minus_and_plus_lines();
         // In color_only mode, raw_line's structure shouldn't be changed.
         // So it needs to avoid fn _handle_diff_header_header_line
         // (it connects the plus_file and minus_file),

src/paint.rs

@@ -126,6 +126,9 @@ impl<'p> Painter<'p> {
     }
 
     pub fn paint_buffered_minus_and_plus_lines(&mut self) {
+        if self.minus_lines.is_empty() && self.plus_lines.is_empty() {
+            return;
+        }
         paint_minus_and_plus_lines(
             MinusPlus::new(&self.minus_lines, &self.plus_lines),
             &mut self.line_numbers_data,