git_catchup.py

#!/usr/bin/env python3

import git

def checkout_except(repo: git.Repo, target: str, base: str):
    common = next(iter(repo.merge_base(target, base)))
    target_files = repo.commit(base).diff(target)
    base_files = set(repo.git.diff(common, base, name_only=True).splitlines())
    for d in target_files:
        if d.a_path in base_files:
            continue

        change_type = d.change_type

        if change_type not in {'A', 'D', 'M', 'R'}:
            raise ValueError(f"Unknown change type: {change_type}")

        if change_type in {'D', 'R'}:
            repo.index.remove(d.a_path, working_tree=True)

        if change_type in {'A', 'M', 'R'}:
            repo.git.checkout(target, '--', d.b_path)
            repo.index.add(d.b_path)


if __name__ == "__main__":
    import sys
    import os
    import argparse

    script_name = os.path.basename(__file__)

    parser = argparse.ArgumentParser(description='Check out all files that are not '
                                                 'changed between two branches')
    parser.add_argument("base", nargs='?', default='HEAD', help="base branch (default HEAD)")
    parser.add_argument("target", nargs='?', default='master', help="target branch (default master)")
    args = parser.parse_args()

    if not args.base or not args.target:
        parser.print_usage()
        sys.exit(1)

    checkout_except(git.Repo(), args.target, args.base)