Merge branch 'ah/rebase-keep-base-fix' into maint

"git rebase --keep-base <upstream> <branch-to-rebase>" computed the
commit to rebase onto incorrectly, which has been corrected.
source: <20220421044233.894255-1-alexhenrie24@gmail.com>

* ah/rebase-keep-base-fix:
  rebase: use correct base for --keep-base when a branch is given