branch split: Support moving active branch down

[abhinav]

Given a branch X with two commits:

main --> abc --> def (X)

If we attempt to split at abc, the branch split dialog requires that abc get a name that is not X, and that X must remain at the top. This is not always desirable. Sometimes you want:

main --> abc (X) --> def (New)

We should support this workflow with branch split.

[abhinav]

closing in favor of #411