feat: Add --init option to update command

[twpayne]

Fixes #1427.

[twpayne]

@felipecrs A quick heads up on this. This PR fixes the immediate issue, but there's a more subtle underlying problem that you as an advanced user of chezmoi might encounter.

Very roughly, chezmoi's reading config code looks like this:

1. Set defaults.
2. Read config from user.
3. Compute extra config variables derived from user config (e.g. guess workingTree from sourceDir if workingTree is not explicitly set, refs feat: Add working tree config option #1459).

This --init flag to chezmoi update only performs step 2 above and does not perform step 3. This means that if you're changing directories or removing template variables (and maybe other things) then the config used by chezmoi update --init might differ from the config used by the next invocation of chezmoi update.

In practice, as long as the config changes are small this should not be a problem, but I know that you occasionally make big config changes (e.g. #1430), so I wanted to warn you.

[felipecrs]

Interesting, I will have this in mind. Thanks a lot for this new feature!