sagemathgh-38676: Fix a bug in retrofit-worktree.sh

    
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as shown in https://github.com/sagemath/sage/actions/runs/10919536449/jo
b/30307397930?pr=38659, where the error message
```
[sagemath_doc_html-none] [spkg-install] [installat]
/sage/src/doc/en/installation/linux.rst: WARNING: document isn't
included in any toctree
```
is due to the file `src/doc/en/installation/linux.rst` that is not
cleaned up in the "new" branch being tested.

The problem is that the script does not clean the worktree well enough
when checking out the "new" branch.

Test: https://github.com/sagemath/sage/actions/runs/10934323900/job/3035
4225389?pr=38659, an excerpt from which  is
```
sagemath#30 6.340 + cd worktree-pre
sagemath#30 6.340 + git add -A
sagemath#30 6.989 + git commit --quiet --allow-empty -m old -a
sagemath#30 7.071 + git tag -f old
sagemath#30 7.073 + git checkout -f new
sagemath#30 7.106 Previous HEAD position was 98054dd old
sagemath#30 7.106 HEAD is now at b05f634 new
sagemath#30 7.107 + git clean -fd
sagemath#30 7.159 Removing src/doc/en/installation/linux.rst
sagemath#30 7.159 + git status
sagemath#30 7.243 HEAD detached at new
sagemath#30 7.243 nothing to commit, working tree clean
```

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- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#38676
Reported by: Kwankyu Lee
Reviewer(s): Matthias Köppe

.ci/retrofit-worktree.sh

@@ -43,4 +43,4 @@ git worktree add --detach $WORKTREE_NAME
 rm -rf $WORKTREE_DIRECTORY/.git && mv $WORKTREE_NAME/.git $WORKTREE_DIRECTORY/
 rm -rf $WORKTREE_NAME && ln -s $WORKTREE_DIRECTORY $WORKTREE_NAME
 if [ ! -f $WORKTREE_NAME/.gitignore ]; then cp .gitignore $WORKTREE_NAME/; fi
-(cd $WORKTREE_NAME && git add -A && git commit --quiet --allow-empty -m "old" -a && git tag -f old && git checkout new && git status)
+(cd $WORKTREE_NAME && git add -A && git commit --quiet --allow-empty -m "old" -a && git tag -f old && git checkout -f new && git clean -fd && git status)