q: How can I see diffs in bottom files by https://github.com/lifo/docrails/compare link, when there are too many files
a: I am afraid we cannot show large diffs. You can see it locally though. /Petros Amiridis (GitHub Staff)/

Поэтому в помощь форк docrails. Инструкция по форкам
А затем берем дифф по отдельному файлу: git diff fa159d317611d1fe5b48 — guides/source/getting_started.textile >> gettin.diff