Automerge: [Github] Simplify Getting Changed Files in Code Formatting Workflow (#133023)

This patch changes getting changed files in the pr code format job to
just checking out the previous two commits (the merge commit and its
porent, the current commit latest in main), which allows us to just diff
the merge commit. This means we do not have to checkout the ref through
the merge base, which should save approximately a minute per job (or
much more in some cases where the PR is particularly out of date).

Author: boomanaiden154