[Github] Use three dot diff for darker in code format action (llvm#74704)

Using a two dot diff allows changes made in main after the merge base to
show up in the formatting diff. Using a three dot diff fixes this and
ensures that only changes made in the source branch (branch from the PR
author) will get passed along to the formatter.

Without this, issues like llvm#73873 occur.

Author: boomanaiden154

llvm/utils/git/code-format-helper.py

@@ -185,7 +185,7 @@ def format_run(
             "--check",
             "--diff",
             "-r",
-            f"{args.start_rev}..{args.end_rev}",
+            f"{args.start_rev}...{args.end_rev}",
         ] + py_files
         print(f"Running: {' '.join(darker_cmd)}")
         self.darker_cmd = darker_cmd