rebase -i: do not leave a CHERRY_PICK_HEAD file behind

rebase -i: do not leave a CHERRY_PICK_HEAD file behind

When skipping commits whose changes were already applied via `git rebase
--continue`, we need to clean up said file explicitly.

The same is not true for `git rebase --skip` because that will execute
`git reset --hard` as part of the "skip" handling in git-rebase.sh, even
before git-rebase--interactive.sh is called.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>