Narrow better the inputs diff on build failed

[viric]

For example, in http://hydra.nixos.org/build/5089682, something affected gcc 4.7 and made it fail.

Nevertheless, instead of showing the diff between 1) the last version of nixpkgs where gcc 4.7 built fine and the failing build, it shows 2) the last nixpkgs change that caused a succesful gcc rebuild and the failing nixpkgs. The 2) is over 250 changes, while 1) would be a very specific diff.

I thought I had file this issue before, but I can't find it now.