[Github] Fetch an additional commit for docs CI on PRs

There still seem to be issues with the files changed step taking a
significant amount of time on PRs. This seems to be occurring on PRs
with one commit and not on PRs with more than one commit which is why I
didn't catch this nuance before. Either way, fetching an additional
commit seems to fix the issue. Requires a little bit of hackiness due to
the fact that you can't do math in the github actions variables.

Will be monitoring this over the next little bit to see if this actually
fixes the problem.

CI stuff is annoying sometimes.