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.
No clue if that's a solution for you on iOS but it's a great feature.
Unfortunately, iOS browsers tends to be just reskins of Safari (because it's required by Apple).
The original Readability was a bookmarklet that worked in any browser.
iOS Safari Reader mode - https://i.imgur.com/nDnBSAM.jpeg
iOS Safari - https://i.imgur.com/AhtWv9G.jpeg