There’s no obvious UI to do this, but there is a way to construct the right URL to go to page one of the commit log.
Suppose we want to find the first commit of the dotnet/coreclr repository. First take a note of the number of commits in the repository: it is currently 16,634. Now go to the list of commits, and click “Older” once. The URL will be something like this: https://github.com/dotnet/coreclr/commits/master?after=872095a758a3a6191a9798c94a98e8d1e16b2254+34
Notice the +34 part. That’s how many commits are skipped. Change that to 16,634 minus 1 minus 35 per page, that gives us this URL, which takes you right to the first page of the coreclr commit history.