manip2.py

#!/usr/bin/env python3

import os
import re
import sys
import zlib
import base64
import subprocess

from hashlib import sha1
from pathlib import Path
from itertools import chain

def check_output(*a, **kw):
    if 'encoding' not in kw:
        kw['encoding'] = 'utf-8'

    print(a)
    result = subprocess.check_output(*a, **kw)
    #print(result)
    return result

def current_commit():
    return check_output(['git', 'log', '-1', '--pretty=format:%H']).strip()

def parent_commit():
    return check_output(['git', 'log', '-1', '--pretty=format:%P']).strip()

def raw_commit(hash_):
    path = Path('.git', 'objects', hash_[0:2], hash_[2:])
    if path.is_file():
        with open(path, 'rb') as f:
            raw = zlib.decompress(f.read())
            #print(raw)
            return raw[raw.index(b'\0')+1:]

def write_tree():
    return check_output(['git', 'write-tree']).strip()

def rev_parse_head():
    return check_output(['git', 'rev-parse', 'HEAD']).strip()

def hash_commit_body(body):
    output = check_output(['git', 'hash-object', '-t', 'commit', '--stdin'], input=body, encoding=None).decode()
    return output.strip()

def apply_commit_body(body):
    return check_output(['git', 'hash-object', '-t', 'commit', '--stdin', '-w'], input=body, encoding=None).decode()

if __name__ == '__main__':
    parent = parent_commit()
    current = current_commit()
    body = raw_commit(current)
    #check_output(['git', 'reset', '--hard', parent])
    tree = write_tree()
    new_parent = rev_parse_head()
    #print({'tree':tree,'parent':parent,'new_parent':new_parent})
    lines = []
    headers = True
    for line in body.split(b'\n'):
        if headers:
            #if line.startswith(b'tree '):
            #    line = b'tree %s' % tree.encode()
            if line.startswith(b'parent '):
                line = b'parent %s' % new_parent.encode()
            elif line == b'':
                lines.append(b'magic_%s placeholder' % new_parent.encode())
                headers = False

        lines.append(line)
    print('modified commit')
    merged = b'\n'.join(lines)
    sha1 = hash_commit_body(merged)
    print(merged.decode())
    print(apply_commit_body(merged))
    check_output(['git', 'reset', '--hard', sha1])
    check_output(['git', 'log', '--pretty=format:%ae'], encoding=None)