Add better UI/UX for dealing with rebase steps

[eamodio]

Once an interactive rebase is started, GitLens doesn't provide any help/guidance for progressing through the rebase steps. Investigate adding a new view or a new node to the commits/repositories view to help walk through the rebase.

[dlech]

Not sure what the intention of this issue is, but one thing that is missing in the current UI is the ability to add extra nodes. This is useful to basically do a rebase and cherry pick at the same time.

Also, if there was a way to add nodes then implementing the x/exec option would be useful too.

[eamodio]

@dlech Can you open a new issue for those? And please add more details to what you mean by adding extra nodes -- I'm not sure I'm following.