Diff after a force push should use range-diff, not an absolute diff

[jyn514]

Consider for example this force-push: https://github.com/rust-lang/rust/compare/86d182ebd0cf9900f32e1f58addc1fc21f9b791b..f8934505dcde03c1db784bde05b81c926bbc1714

This shows a diff of 1,275 changed files with 20,128 additions and 8,906 deletions. Almost all of these changes were from rebasing over the master branch; finding the changes which aren't related to that is basically impossible. GitHub should instead show a range diff of the changes, which is much more readable:

$ git range-diff 86d182ebd0cf9900f32e1f58addc1fc21f9b791b~..86d182ebd0cf9900f32e1f58addc1fc21f9b791b f8934505dcde03c1db784bde05b81c926bbc1714^..f8934505dcde03c1db784bde05b81c926bbc1714 --
1:  86d182ebd0c = 1:  f8934505dcd Remove liballoc unneeded explicit link

From this you can clearly see there are no changes other than the rebase.