Ability to "split" commits

Similar to how you can stage code on a per-line basis, it would be nice to be able to go through a commit log and "split" it into multiple commits. For example, if I accidentally commit a file that has two fixes, it would be nice to go in to the commit log, and select which lines you want to be pulled out of the original commit and added as a second commit.

Understandably, this would modify the commit history - similar to appending a commit or rebasing. It might be good to have a similar "warning" for the user about the history being modified.

I don't know if Git proper has the ability to do this, but I have been able to achieve this same end result through creative use of the SourceTree "reverse lines/hunk/file", "append previous commit" and "rebase children interactively"...but it would be nice to have some sort of UI-based way of doing this.