Edit run_onchange_03-install-packages.tmpl: reduce number of times chezmoi data executes

[zipperer]

Claims I believe (each of which could be false and thereby make this commit wrong):

• each call to get_config returns the same information (i.e. for each call the return value is the same)
• filter(lambda p: matches_features(p, get_config()), to_install), calls get_config() for each entry in to_install
• get_packages computes data, and data contains what get_config returns

Based on those claims, I propose:

• get_packages returns both packages and config
• in filter(lambda p: matches_features(p, get_config()), to_install), replace get_config() with config, where config was computed once before and is reused for each package p

[JoshMock]

Good idea. This only runs every few days when I run chezmoi update, but an optimization is always a win, especially for a repo that's all about having fun with it. 😎

I'd personally probably memoize it using a global instead. Haven't run this so I don't know if it's perfect, but something like:

chez_config = None

def chezmoi_data():
    if not chez_config:
        result = subprocess.run(["chezmoi", "data"], capture_output=True)
        chez_config = json.loads(result.stdout)
    return chez_config

def get_config():
    return chezmoi_data()["chezmoi"]["config"]["data"]

def get_packages():
    return chezmoi_data()["packages"]

That way it just fetches it once and stores it in memory for the remainder of the process lifetime, and I don't have to return a tuple, which almost always (for me) requires me to go look at the function and figure out what it's actually returning. 😆