revert: allow single-pick in the middle of cherry-pick sequence

revert: allow single-pick in the middle of cherry-pick sequence

After messing up a difficult conflict resolution in the middle of a
cherry-pick sequence, it can be useful to be able to

	git checkout HEAD . && git cherry-pick that-one-commit

to restart the conflict resolution. The current code however errors out
saying that another cherry-pick is already in progress.

Suggested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>