[clang-format] Use proper flags for git diff-tree (llvm#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.

Author: boomanaiden154

clang/tools/clang-format/git-clang-format

@@ -419,7 +419,7 @@ def compute_diff(commits, files, staged, diff_common_commit):
     if len(commits) == 2:
         git_tool = "diff-tree"
         if diff_common_commit:
-            commits = [f"{commits[0]}...{commits[1]}"]
+            extra_args += ["--merge-base"]
     elif staged:
         extra_args += ["--cached"]