sequencer (rebase -i): skip some revert/cherry-pick specific code path

sequencer (rebase -i): skip some revert/cherry-pick specific code path

When a cherry-pick continues without a "todo script", the intention is
simply to pick a single commit.

However, when an interactive rebase is continued without a "todo
script", it means that the last command has been completed and that we
now need to clean up.

This commit guards the revert/cherry-pick specific steps so that they
are not executed in rebase -i mode.

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