[llvm] [Github] Use API to fetch PR diff for docs action (PR #70001)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Is this essentially a revert of your previous patch?

No, just switching up the approach. In the previous patch I attempted to get it working this way and then switched to pulling the number of commits in the PR (naively thinking that would work) to avoid the API call, which is what landed. That didn't work however (at least in some cases), and I'm reasonably confident that forcing usage of the API will, hence the patch.

https://github.com/llvm/llvm-project/pull/70001