Feat: Automatically Detect Stale Branches

[harrisoncramer]

As pointed out by @levouh here #25 (comment)

It'd be best if this library could detect when your current or target branch is out of date. I'm thinking that the best solution here is to prompt you to re-pull those branches to your local system prior to running the reviewer. Then you could get a prompt popup (You are out of date, would you like to pull down those branches?) and then the reviewer would do this automatically.

There could be some complexity if either pull fails (for instance if you have to rebase on either branch). However in those cases we could probably just prompt the user to pull down manually.

Running the reviewer against the remote branches directly is overly complex because it would likely involve comparing against temporary buffers and would break the basic functionality of how both Delta and Diffview currently work.