[llvm] [Github] Fetch all commits in PR for code formatting checks (PR #69766)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Now just fetching the number of commits in the PR. This makes the changed files step fast (couple of seconds) and the changed files step still ensures the relevant history is there so that the script can run `git clang-format` properly.

Tested against my fork and everything was working well there.

https://github.com/llvm/llvm-project/pull/69766