sequencer: future-proof read_populate_todo()

sequencer: future-proof read_populate_todo()

Over the next commits, we will work on improving the sequencer to the
point where it can process the todo script of an interactive rebase. To
that end, we will need to teach the sequencer to read interactive
rebase's todo file. In preparation, we consolidate all places where
that todo file is needed to call a function that we will later extend.

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