filter.py

#!/usr/bin/python

import sys
import subprocess

parents = filter(lambda p: p!='-p', sys.stdin.readline().split() )
output = []

for parent in parents:
    result=subprocess.check_output(["git", "rev-list", parent] + map(lambda p: "^%s"%p, filter(lambda p: p != parent, parents)))
    if result:
        output.append(parent)

if len(output) == 0 and len(parents) != 0:
    output.append(parents[0])

print(" ".join(map(lambda p: "-p %s"%p, output)))