[clang-format] Use proper flags for git diff-tree (#155247)

From local testing, git diff-tree does not support three dot diffs
correctly, instead expecting the --merge-base flag to be passed along
with two commits. From my reading, the documentation
(https://git-scm.com/docs/git-diff-tree) also confirms this. This patch
updates the git-clang-format script to be correct.

I don't think we ever ran into this issue before because we never ended
up using it. For the PR code format job I believe we would just
explicitly pass the merge base, completely bypassing the problem.

NOKEYCHECK=True
GitOrigin-RevId: f09986aab575b41e2e210d937a46dfce82eceebc
1 file changed