Have an option to only show terraform diffs in Github comment

[bmckerry]

Describe the solution you'd like
Right now, having the action do a run-all plan/apply in a high-level dir outputs a lot of unhelpful text - in our case, we use this action to run a plan on one of our entire repos, which outputs >2000 lines of text, <50 of which are the actual change plan.

It'd be nice to have an only_output_diffs option which would only output everything between Terraform will perform the following actions: and Plan: X to add, X to change, X to destroy.

Describe alternatives you've considered
I can try adding a filter to the output text on our end, but not totally sure how I'd go about doing that since the PR commenting happens in the terragrunt-action script.

EDIT: after poking around a little more, I might be able to add this on our end via an INPUT_POST_EXEC_ env var - would be neat to see this as a built-in feature, though.

Additional context
n/a

[bmckerry]

For now, I worked around the issue by adding this to the GHA env:

INPUT_POST_EXEC_0: |
            IFS=$'\n'
            local -r log_file="${terragrunt_log_file}"
            local terragrunt_log_content=$(cat "${log_file}")
            local terragrunt_output=$(clean_colors "${terragrunt_log_content}")
            local diffs_only_output=""
            local concat_line=false
            for line in $terragrunt_output; do
              if [[ $line == "Terraform used the selected providers to generate the following execution" ]]; then
                concat_line=true
              fi
              if $concat_line; then
                diffs_only_output="${diffs_only_output}\n${line}"
              fi
              if [[ $line == *"Plan:"* ]]; then
                concat_line=false
              fi
            done
            echo "terragrunt output:"
            echo -e $diffs_only_output 2>&1 | tee "${terragrunt_log_file}"
            IFS=$' \t\n'

Maybe there's a better way to handle it, my bash isn't great but it seems to work