sequencer: factor out part of pick_commits()

sequencer: factor out part of pick_commits()

This simplifies the next commit. If a pick fails we now return the error
at the end of the loop body rather than returning early, a successful
"edit" command continues to return early. There are three things to
check to ensure that removing the early return for an error does not
change the behavior of the code:

(1) We could enter the block guarded by "if (reschedule)". This block
    is not entered because "reschedlue" is always zero when picking a
    commit.

(2) We could enter the block guarded by
    "else if (is_rebase_i(opts) &&  check_todo && !res)". This block is
    not entered when returning an error because "res" is non-zero in
    that case.

(3) todo_list->current could be incremented before returning. That is
    avoided by moving the increment which is of course a potential
    change in behavior itself. The move is safe because none of the
    callers look at todo_list after this function returns. Moving the
    increment makes it clear we only want to advance the current item
    if the command was successful.

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