Allow to use `.chezmoi.stdin` in `execute-template` command

[felipecrs]

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

I was reporting https://github.com/twpayne/chezmoi/issues/2768, and my first attempt to build a command to reproduce the issue was:

printf "%s\n" "[boot]" "systemd=true" | chezmoi execute-template '{{ .chezmoi.stdin | fromIni | toIni}}'

And, I understand that stdin should actually be used to supply in the template itself.

Describe the solution you'd like

I think it could work:

• If there is an argument, read the template from the argument and pass in the command stdin to the .chezmoi.stdin template variable rather than reading the template from the stdin.

Describe alternatives you've considered

N/A

Additional context

This is just a suggestion, and I'm not sure if it's even a good idea, or if it's even worth implementing. Anyway, if you dislike feel free to close this issue, no hard feelings!

[halostatue]

I think that this is a good idea as it will make it easier to test modify templates. Maybe with a flag of --modify-template or --use-stdin specifically against execute-template commands.

The usual trick of adding {{/* chezmoi:modify-template */}} does not work with execute-template and is, I think, opaque for this purpose.