CI docbuild: Possible merge conflict in `git stash pop`

[mkoeppe]

The use of git stash pop can fail with a merge conflict as seen in
https://github.com/sagemath/sage/actions/runs/10232970523/job/28310739740?pr=38224#step:13:74

[Demon-Sheriff]

@mkoeppe I would like to work on this issue.

[kwankyu]

@Demon-Sheriff Do you understand why it happens? I have no idea.

[kwankyu]

The relevant part of the script:

(A)         (cd doc && git commit -a -m 'new')
            # Wipe out chronic diffs of new doc against old doc before creating CHANGES.html
(B)         (cd doc && \
             find . -name "*.html" | xargs sed -i -e '/This is documentation/ s/ built with GitHub PR .* for development/ for development/' \
                                                  -e '/<link rel="stylesheet"/ s/?v=[0-9a-f]*"/"/' \
                                                  -e '\;<script type="application/vnd\.jupyter\.widget-state+json">;,\;</script>; d' \
                                                  -e 's;#L[0-9]*";";' \
             && git commit -a -m 'wipe-out')
            # Since HEAD is at commit 'wipe-out', HEAD~1 is commit 'new' (new doc), HEAD~2 is commit 'old' (old doc)
(C)         .ci/create-changes-html.sh $(cd doc && git rev-parse HEAD~2) doc
            # Restore the new doc with changes made in create-changes-html.sh but dropping changes by "wipe out"
(D)         (cd doc && git stash -q && git checkout -q -f HEAD~1 && git stash pop -q)     

After (A), we have commits "old" < "new"

After (B), we have commits "old" < "new" < "wipe-out"

After (C), we have some changes on "wipe-out". I want to keep these changes but on "new".

So in (D), git stash -q stashes the changes, and git checkout -q -f HEAD~1 force-checks out "new" branch, effectively removing changes in "wipe-out", and git stash pop -q is supposed to recover the stashed changes.

[kwankyu]

I think I can fix it now. Some radical change is necessary.

[kwankyu]

Testing the fix with #38468.