vim-patch:9.2.1115: ml_updatechunk() rescans all chunks on every line update (#41949)

Problem:  ml_updatechunk() only resumes its chunk scan for strictly
          consecutive line insertions; for line updates (e.g.
          ":substitute") and any other non-consecutive access it
          restarts the scan from the first chunk, which is slow on
          large buffers.
Solution: Resume the forward scan from the cached chunk position
          whenever the wanted line is at or after it, falling back to a
          full scan only when moving backwards or into another buffer,
          and invalidate the cache only when the chunk table is
          changed.

The static resume cache was previously used only for the strictly
consecutive ML_CHNK_ADDLINE fast path. This commit generalizes it to cover all
update types: every mutation leaves the cache pointing at the last
touched chunk with a correct start line; the exits that move chunks
around (split, collapse, reset) clear it, while the ones that leave the
chunk table intact keep it, including the ML_CHNK_DELLINE exit that only
decrements a line count. This turns repeated in-order updates (":s", line
insertions and deletions such as ":g/pat/d") from O(chunks) per call
into amortized O(1).

With callgrind the substitute workload drops 1.8% in total instructions and
ml_updatechunk() self cost drops 63%, granting a 10% speed boost on
":%s/\<int\>/int32_t/ge" in a buffer containing src/*.c

closes: vim/vim#21309

https://github.com/vim/vim/commit/8692965f52a8e13084a6cb4d726ddc5f3b25c2c6

Co-authored-by: Julien Voisin <julien.voisin@dustri.org>

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