Allow diff to include crds

[obirhuppertz]

Goal

This PR allows usage of helm template flag --include-crds during a helm diff upgrade.

Before

When using helmfile with helm diff upgrade command you have to use post-renderer to get the fully rendered manifest and allow rendering of all resources including CRDs that are deployed via a sub-chart i.e. kyverno.

Use Case

In order to be able to move a way from post-renderer and use kustomization builtin support of helmfile without breaking the manifest that you can get via helm get manifest -n namespace release it is needed to allow rendering the included crds if the chart decided to render these via a dependency.

It also makes three-way-merge more usefull as you now also get the diff for included crd changes.

After

Merging this PR into an upcoming release would allow anyone to be able to catch changes to included crd changes during a helm diff upgrade.

Breaking Changes?

None to be expected, the default is set to false which reflects the current behavior. You have to pass --include-crds excplitictly to also render included crds when calling template before diff when using three-way-merge
We used helm diff upgrade command in conjunction with helmfile with kustomize postrenderer.

Follow up

helmfile maintainers could add this flag to helmfile diff command in future versions. Using just the patched version would need you to add - "--include-crds" to helmDefaults.diffArgs array.

(at)Others who read this - please upvote if you find this PR useful.

[AMOKANDY]

Fine!. But please add the new option to “readme.md”.