Check whether the newly pulled revisions are marked with a special keyword

post-merge.py

#!/usr/bin/env python3

import subprocess
import sys

PATTERN = 'MY-SPECIAL-COMMIT-MARKER'

with subprocess.Popen(['git', 'reflog'], stdout=subprocess.PIPE) as proc:
    lines = proc.stdout.readlines()
new, old = [l.decode().split()[0] for l in lines[:2]]

with subprocess.Popen(['git', 'log', '--grep', PATTERN, '%s..%s' % (old, new)], stdout=subprocess.PIPE) as proc:
    output = proc.stdout.read()

if output:
    print('Found', PATTERN, file=sys.stderr)
    # you action here
else:
    print('Nothing found...', file=sys.stderr)