Support --empty flag for handling empty commits during rebase

[fulghum]

Today, when rebasing commits, if one of the commits becomes empty, the rebase will fail. With Git, you can use the --empty parameter to specify how you'd like Git to handle commits that become empty during a rebase. The keep and drop actions are likely the most useful for customers in Dolt's context.

It probably also makes sense to look at the --keep-empty and --no-keep-empty flags while looking at this work. (Note that these flags apply to commits that start empty before the rebase, where as the --empty param above applies to commits that become empty during a rebase.)