- Remove REBASE_MODE_NORMAL. It is not the "normal" mode anyway, rather a legacy
mode; we have removed support for it in eb0f7e3d02, so there's no point in
representing it in the enum.
- Remove distinction between REBASE_MODE_REBASING and REBASE_MODE_INTERACTIVE;
these are the same now.
- Rename StatusCommands.IsInInteractiveRebase to IsInRebase.
- Remove StatusCommands.RebaseMode; use StatusCommands.IsInRebase instead.