Find the common ancestor for the style checking

This allows the style checker to only look at the changes done in the
current branch, and not the other commits that were done in the base
branch.

.github/workflows/style.yml

@@ -23,5 +23,5 @@ jobs:
     - name: Check style
       run: |
         clang-format -version
-        scripts/fix_style.py --dry-run --base "$(git merge-base origin/master $GITHUB_SHA)"
+        scripts/fix_style.py --dry-run --base origin/master
         scripts/check_header_guards.py

scripts/fix_style.py

@@ -43,6 +43,8 @@ def warn(changed_lines):
 		result = subprocess.call(["clang-format", "-Werror", "--dry-run"] + ["--lines={}:{}".format(start, end) for start, end in changed_lines[filename]] + [filename]) or result
 	return result
 
+def get_common_base(base):
+	return subprocess.check_output(["git", "merge-base", "HEAD", base]).decode().strip()
 
 def main():
 	import argparse