BUG: Fix RangeIndex.get_indexer for decreasing RangeIndex
- [X] closes #28678
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

@jschendel you will need to fix the conflicts now I merged the other PR lgtm. @jreback : green Thanks!