rebase -i: Enable options for pick and reword

pick and reword are atomic to-do list commands in the sense that they
open a new task which is closed after the respective command is
completed. squash and fixup are not atomic. They create a new task which
is not completed until the last squash or fixup is processed.

Remove the `test -z "$opts"` blockade in front of the pick and reword
handling code.

Enable the options `--signoff`, `--gpg-sign`, `--reset-author`,
`--amend`, `--edit` for the to-do list commands pick and reword. The
option `--file` is not yet supported because `do_next` cannot handle
option arguments at the moment. It is true that edit is one of the
atomic commands but it displays hash information when the rebase is
stopped and some options rewrite the picked commit which alters that
information. squash and fixup still do not accept user options.