Enable reverse comparing between branches

**Is your feature request related to a problem? Please describe.**
Currently (as far as I can tell) the compare function always shows the diffs with the branch that is currently checked out. This means that to see the diffs as they would in a PR, you have to checkout the branch you are merging to, instead of the one you are working on. As I like to see the PR I'm about to submit before committing/opening the PR, this is something I do fairly regularly and having to switch branches to do it is somewhat awkward.

**Describe the solution you'd like**
An option to flip the base of the diff when viewing patches.

**Describe alternatives you've considered**
- A shortcut to switch branches but that wouldn't really work in you don't have a clean working tree, which is most of the time in my experience.
- Changing the default to be flipped compared to what it is now, but this would be a breaking change compared to what it is now and I'm not sure how many people this would piss off. I guess this might be an option if the extra option is not desirable, but it seems a little excessive to me.
**Additional context**
Sounds like a not too complicated feature although I'm not familiar with the code base. If there is interest in this feature and someone willing to help guide/review work a little bit, I'd be open to making a PR for this myself.

Bắt đầu bằng cách xác định hàm compare và điểm vào để xem patch, sau đó theo dõi cách branch đã checkout trở thành diff base. Hoàn tất khi một tùy chọn rõ ràng có thể chuyển base đó mà không yêu cầu chuyển branch; thêm hoặc cập nhật coverage nếu hành vi compare hiện có đã có test.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.