Diff stats in PR tab bar

[silverwind]

Feature Description

It would be nice if we could show a diff stat on the right side of the PR tab bar.

Screenshots

[hiifong]

How to calculate the number of squares is a problem?

[silverwind]

We don't have squares in the file header either, so a "fluid" bar like there will do.

[hiifong]

Which method is called by default when the path {user}/{repo}/pull/{index} is requested,

[silverwind]

Keep note that it should show an all tabs, e.g. the relevant context data needs to be added to GET /files and /commits as well.