rebase: drop get_replay_opts()

[agrn]

The builtin rebase stores its option to a struct rebase_options, but the sequencer wants a struct replay_opts. So, before starting an interactive rebase, the former must be converted to the latter. That’s what get_replay_opts() is all about. Instead of that, it could be cleaner to remove the redundancies between struct rebase_options and struct replay_opts, and then to get rid of get_replay_opts().

I already brought it up there: https://public-inbox.org/git/[email protected]/

I already started some work on this, but as ra/rebase-i-more-options adds, well, more options to rebase -i, and thus is touching get_replay_opts(), I prefer to delay this a bit.