[llvm] [Github] Fetch number of commits in PR for docs action (PR #69763)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> LGTM. If the speed is still an issue, the other solution would be to have separate workflow files for the clang and llvm docs.

Thanks for the review! I don't think the speed was a huge issue, but it's definitely better to not waste tens of minutes doing essentially nothing. After this I think the speed on PRs should be more than fine (sub ten minutes). If we still need more speed, I'll work on splitting jobs/throwing all the dependencies into a container image.

https://github.com/llvm/llvm-project/pull/69763