Implement an interactive `git split` command

[bcongdon]

As discussed in #48.

Note: Depending on how this is implemented, we may need a git stage command as a prerequisite for this

[arxanas]

Note: I tried using the equivalent of git merge-tool, and while it seems powerful, I think it's sufficiently difficult to use without training that we shouldn't use it for splitting/staging commits by default.

Re git stage in particular, I suppose that ideally we wouldn't have that command at all and we would never touch the staging area ourselves, and instead rely on commands like git split and some kind of git commit --interactive command to accomplish partial commits. But you already get the idea about the interface for all three of these being the same.