Image URLs pointing at a GitHub "blob" page, such as
https://github.com/<owner>/<repo>/blob/<ref>/<path>, serve an HTML
page rather than the raw image bytes, so they render as broken images
in markdown.
Add a canonicalizeGithubImageUrl helper that appends ?raw=true to such
URLs, which makes GitHub redirect to the raw content. URLs already
pointing at the raw content, via ?raw=true or raw.githubusercontent.com,
are left unchanged, as are non-GitHub and non-URL sources.
Wire the helper into the markdown image loop so external image URLs are
canonicalized while relative paths keep resolving against the repo's raw
endpoint.
andreolf
8bb45aa11d