The file has diagrams that are meant to be viewed on a screen with enough space to leave long lines unbroken. If you attempt to get things to fit onto a single line by turning the phone sideways, it just zooms the text, instead of reflowing the text onto a larger line width. The problem exists in both the default rendering and the reader view.
Opening in iBooks essentially prints the text file to a PDF, which defaults to US Letter paper size, which has the effect of making line widths large enough for most 80-character text files to fit without awkward mid-line soft breaks.
The only other solution I know of is to manually zoom the page out to 50%. Luckily the zoom setting is saved by domain, so in this case if you want all raw githubusercontent files to view zoomed out iOS will remember that, but on domains where it’s a mix of text and HTML it’s more annoying.
Firefox reader mode is nice for taking various webpages, whether it's a news article with bloated JS, a wiki page, simple plaintext etc and giving it a common presentation that you can configure to your liking.
No clue if that's a solution for you on iOS but it's a great feature.
