gh-38468: Fix buggy doc diff script

    
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Still after #38425, the script generating CHANGES.html needs
improvement.

(1) Fixes #38467

test: https://github.com/kwankyu/sage/actions/runs/10257146932/job/28377
629063?pr=63
test:  https://doc-pr-63--sagemath-test.netlify.app/changes

(2) As seen in https://doc-pr-38339--sagemath.netlify.app/changes (hunk
1), the anchor may be planted in a line very far from where the diff is.

test: https://doc-pr-65--sagemath-test.netlify.app/changes  (hunk 1)

(3) As seen in https://github.com/sagemath/sage/actions/runs/10235554395
/job/28316513988?pr=38449, the script may fail (and hence doc build
github workflow as well) by a bug (I failed to consider that git diff
counts lines from 1).

test: https://github.com/kwankyu/sage/actions/runs/10257125674/job/28377
566493?pr=64
test: https://doc-pr-64--sagemath-test.netlify.app/changes (warning:
slow loading)

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### ⌛ Dependencies

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URL: #38468
Reported by: Kwankyu Lee
Reviewer(s): Matthias Köppe