excessive git repo size due to API docs generation keeping history

[evaleev]

git for-each-ref --format='%(refname)' |
while read branch
do
	size=$(git rev-list --disk-usage --objects HEAD..$branch)
	echo "$size $branch"
done |
sort -n

produces

...
1718504 refs/remotes/origin/bspline
4084920 refs/remotes/origin/mshf_stable
18666808 refs/remotes/origin/tr1
364196113 refs/remotes/origin/gh-pages

as a result fresh checkout takes minutes + repo takes gigabytes unless shallow-cloned (but no developer does it and FetchContent does not support shallow-cloning either) ... this will only continue to get worse.

solution: overwrite the contents of gh-pages branch every time, see https://github.com/TESSEorg/ttg/blob/master/.github/workflows/cmake.yml#L157