[Github] Fetch through merge base in code formatting action (#72020)

This commit adds another step to the Github workflow that runs the code
formatting check to fetch through the merge base. This ensures that the
necessary history is present to find the changed files and also to run
clang-format over. This change massively increases the speed of the
action (~10 minutes down to ~2 minutes in most cases from my testing)
and also increases the reliability significantly.