As Maja suggested, links to multiple commits will now be rendered using the whitelistedgeneric onebox
https://github.com/discourse/onebox/commit/4103d115c199458a9a9368e5962dd20e674abf52
Unfortunately the opengraph data which github provides for these URLs is generic - it will just show the repo name/description. However, I still think this is less confusing than arbitrarily rendering the first commit.
A brand new onebox for these “multi-commit” links would still be #pr-welcome, if someone would like to try it.