Problem:
win_line() falls into infinite loop when a diff window has top filler
above its first visible buffer line, that first visible buffer line is a
closed fold, and the folded line uses normal non-empty foldtext.
Solution:
Allow flushing pending diff filler rows even when the underlying buffer
line is folded with foldtext.
AI-assisted: Codex
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
f2cc0a249d