Problem: do_sub() only checks the timeout limit after finishing a line.
A pathological regex will run on a single line input unbounded
until the compute is completed.
Solution: Pass the timeout limit to `vim_regexec_multi()` so the
computation on the regex engine is bounded per-line.
Signed-off-by: XiaowenHu96 <me@xiaowenhu.com>
(cherry picked from commit bd73d8c011)