I forked tobie/specref on github.com, and tried to review some changes for biblio.json. This is a big file (ca. 5MB), and the diff is not shown. SourceTree shows diffs just fine for other json files, so the file type does not seem to be the problem. It seems the size is the killer here. I can view the diffs fine with external diff tools (I tried Appel's FileMerge, and Emacs edify mode).