Hey Ryan! Sorry I haven't done a review on this until now.
Very neat idea. It might be worth us exploring this for other types of repositories.
One thing that stands out is that the order of files in the diff are probably not going to be preserved the way that it's currently generating them. Am I right about that? Would you be able to have this keep track of order?

Probably better to define this as its own function, rather than defining inline. Maybe generalize the name, call it _diff_file().
There's a performance hit to defining inline functions (likely won't matter much here, but it's bitten us before), and I could see doing something interesting down the road with having a standardized function that just diffs a single file, and have a standardized way to parallelize all the calls needed to those, simplifying what needs to be done in each implementation.

You'll need double backticks to be valid ReStructuredText.

Our diff generator won't handle the None: part right. You can just include the possibility of this in the description ("This may be None if there's an issue generating the diff. In that case, a warning will likely be emitted.").

Can you explicitly return None? We go for explicit rather than implicit here.
Same applies below.

Blank line in-between statements and blocks.