sequencer.c: factor out a function

sequencer.c: factor out a function

This code is heavily indented and obscures the high level logic within
the loop. Let's move it to its own function before modifying it in the
next commit. Note that there is a subtle change in behavior if the
todo list cannot be reread. Previously todo_list->current was
incremented before returning, now it returns immediately.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>