from unittest.mock import call


def find_missing_git_remotes(remotes_to_check):
    """
    check that each value in a list of strings is an existing git remote
    returns the name of the first missing remote (if any), or None if all
    remotes exist
    """
    git_remotes = call('git remote', output=True).split('\n')
    for remote in remotes_to_check:
        if ':' in remote:
            remote_name, remote_branch = remote.split(':')
        else:
            remote_name = remote
        if remote_name not in git_remotes:
            return remote
    return None