Clean up histedit/rebase output

[zombiezen]

gg histedit and gg rebase pass through a bunch of output from git rebase. While some of it is helpful, it's not organized particularly well. Especially after fixing #21, there's a bunch of output suggesting Git commands where there are more idiomatic gg counterparts that have more safeguards. It would be good to extract the same information, and then always run git rebase with --quiet. This will require a bit of research to see what git rebase outputs under different circumstances (e.g. merge conflicts, finishing the rebase, running continue when there are unmerged paths).