FR: add `--apply` to `merge` and `merge-all`

[Lockszmith-GH]

Is your feature request related to a problem? Please describe.

edit already has --apply, it seems that merge, modifying the source state, should also be able to apply, as the eventual source state will most likely end up a bit different from the home dir.

Describe the solution you'd like

Add --apply option to --merge and --merge-all functionality just like the one for --edit

Describe alternatives you've considered

Running chezmoi apply after the commands works.

[halostatue]

--apply on merge makes a lot of sense, but I think it makes less sense for merge-all -- I often find that when I use merge-all I may not be ready to perform the apply because there are other changes that need to be made, or need to be updated differently.