sequencer (rebase -i): the todo can be empty when continuing

sequencer (rebase -i): the todo can be empty when continuing

When the last command of an interactive rebase fails, the user needs to
resolve the problem and then continue the interactive rebase. Naturally,
the todo script is empty by then. So let's not complain about that!

To that end, let's move that test out of the function that parses the
todo script, and into the more high-level function read_populate_todo().
This is also necessary by now because the lower-level parse_insn_buffer()
has no idea whether we are performing an interactive rebase or not.

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