This fixes two problems with the "amend commit with staged changes" command:
1. Amending to a fixup commit didn't work (this would create a commmit with the
title "fixup! fixup! original title" and keep that at the top of the branch)
2. Unrelated fixup commits would be squashed too.
The added integration test verifies that both of these problems are fixed.
Instead of passing a bunch of different options in
PrepareInteractiveRebaseCommandOpts, where it was unclear how they interact if
several are set, have only a single field "instruction" which can be set to one
of various different instructions.
The functionality of replacing the entire todo file with our own is no longer
available; it is only possible to prepend todos to the existing file.
Also, instead of using different env vars for the various rebase operations that
we want to tell the daemon to do, use a single one that contains a json-encoded
struct with all available instructions. This makes the protocol much clearer,
and makes it easier to extend in the future.
It used to work on the assumption that rebasing commits in lazygit's model
correspond one-to-one to lines in the git-rebase-todo file, which isn't
necessarily true (e.g. when users use "git rebase --edit-todo" at the custom
command prompt and add a "break" between lines).
Add Form field to CustomCommandObjects struct
Write user prompts responses to Form field
Ensure that map keys exists
Add form prompts integration test
Remove redundant index
