mr: run msg_filter scripts without chmod +x

With more modern packaging methods, the executable flag gets stripped
from the files in data. Adjust mr.py to run the scripts with python3
explicitly, doing it like that is actually better than having these
scripts that are not meant to be executed directly with the executable
flag on the filesystem.

Fixes: issue 20

mrhlpr/mr.py

@@ -6,6 +6,7 @@ import json
 import logging
 import os
 import re
+import shlex
 import subprocess
 import sys
 import time
@@ -397,7 +398,7 @@ def run_git_filter_branch(
                 "filter-branch",
                 "-f",
                 "--msg-filter",
-                script,
+                f"python3 {shlex.quote(script)}",
                 "--commit-filter",
                 'git commit-tree -S "$@"',
                 f"{from_ref}..HEAD",