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.

[arxanas]

Note that all of the individual parts to implement this are now present, if anyone wants to give it a try. The interactive change selector already works for git record -i and replacing a commit with multiple commits can be done with the rebase engine.

[claytonrcarter]

Something about my git-split shell script stopped working recently and instead of trying to fix, I decided to take on a new project 😆 so I've started working on a non-interactive version of this. I'll post a PR after I've used it for a bit. Right now the UI is git split <revset> <file> [<file> ...] and it's only extracting entire file diffs. We could extend it with an interactive mode later. I'd also like to eventually be able to specify hunks on the command line, eg split @ file.txt:123 to extract only the part of the diff that intersects with line 123.