edit a git commit without changing the hash

bad_git.sh

#! /bin/bash

d="$(mktemp -d)"

# Make a git repo with one file in it.
mkdir "$d/good"
cd "$d/good"
git init
echo good > file.txt
git add -A
git commit -m "first commit"

# Copy the whole thing.
cp -r "$d/good" "$d/bad"

# Go in the copy and edit the object that holds file.txt. Its path is always
# the same because, unlike the commit object, it doesn't contain any
# timestamps.
# NOTE: The blob object contains its own length, so if we wanted to substitute
# something other than another 4 bytes in there, we'd need to edit the length
# too.
file_hash_path=".git/objects/12/799ccbe7ce445b11b7bd4833bcc2c2ce1b48b7"
chmod +w "$d/bad/$file_hash_path"
cat "$d/good/$file_hash_path" \
  | python2 -c "import zlib, sys; sys.stdout.write(zlib.decompress(sys.stdin.read()))" \
  | sed 's/good/💩/' \
  | python2 -c "import zlib, sys; sys.stdout.write(zlib.compress(sys.stdin.read()))" \
  > "$d/bad/$file_hash_path"

echo "Here's a commit from the good repo:"
echo
git -C "$d/good" show -p

echo
echo "And here's "the same commit" from the bad repo:"
echo
git -C "$d/bad" show -p

@oconnor663 thanks for clarification :)