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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Updated. Apparently just setting `fetch_depth` to the number of commits in the PR makes everything go way more smoothly. Not sure why the changed files action takes so long in the first place, but at least this should be way faster.

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