sequencer: remove overzealous assumption in rebase -i mode

sequencer: remove overzealous assumption in rebase -i mode

The sequencer was introduced to make the cherry-pick and revert
functionality available as library function, with the original idea
being to extend the sequencer to also implement the rebase -i
functionality.

The test to ensure that all of the commands in the script are identical
to the overall operation does not mesh well with that.

Therefore let's disable the test in rebase -i mode.

While at it, error out early if the "instruction sheet" (i.e. the todo
script) could not be parsed.

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