Merge branch 'js/rebase-autostash-detach-fix'

"git rebase --autostash" did not correctly re-attach the HEAD at times.

* js/rebase-autostash-detach-fix:
  built-in rebase --autostash: leave the current branch alone if possible
  built-in rebase: demonstrate regression with --autostash

diff --cc builtin/rebase.c
Simple merge

diff --cc t/t3420-rebase-autostash.sh
Simple merge