cherry-pick: fix --quit not deleting CHERRY_PICK_HEAD

cherry-pick: fix --quit not deleting CHERRY_PICK_HEAD

--quit is supposed to be --abort but without restoring HEAD. Leaving
CHERRY_PICK_HEAD behind could make other commands mistake that
cherry-pick is still ongoing (e.g. "git commit --amend" will refuse to
work). Clean it too.

For --abort, this job of deleting CHERRY_PICK_HEAD is on "git reset"
so we don't need to do anything else. But let's add extra checks in
--abort tests to confirm.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>