diff: Add --no-error-on-diff flag

Add a `--no-error-on-diff` flag to the `diff` subcommand so as to not
exit with an error when there are diffs present. It will still exit with
an error if the configs are not valid, so `kubecfg diff` can be a useful
command to run on a branch of a CI system to show diffs and still
validate the configs.

cmd/diff.go

@@ -22,12 +22,14 @@ import (
 )
 
 const (
-	flagDiffStrategy = "diff-strategy"
-	flagOmitSecrets  = "omit-secrets"
+	flagDiffStrategy  = "diff-strategy"
+	flagNoErrorOnDiff = "no-error-on-diff"
+	flagOmitSecrets   = "omit-secrets"
 )
 
 func init() {
 	diffCmd.PersistentFlags().String(flagDiffStrategy, "all", "Diff strategy, all or subset.")
+	diffCmd.PersistentFlags().Bool(flagNoErrorOnDiff, false, "dont exit with error if there are differences")
 	diffCmd.PersistentFlags().Bool(flagOmitSecrets, false, "hide secret details when showing diff")
 	RootCmd.AddCommand(diffCmd)
 }
@@ -47,6 +49,11 @@ var diffCmd = &cobra.Command{
 			return err
 		}
 
+		c.NoErrorOnDiff, err = flags.GetBool(flagNoErrorOnDiff)
+		if err != nil {
+			return err
+		}
+
 		c.OmitSecrets, err = flags.GetBool(flagOmitSecrets)
 		if err != nil {
 			return err

pkg/kubecfg/diff.go

@@ -50,7 +50,8 @@ type DiffCmd struct {
 	DefaultNamespace string
 	OmitSecrets      bool
 
-	DiffStrategy string
+	DiffStrategy  string
+	NoErrorOnDiff bool
 }
 
 func (c DiffCmd) Run(apiObjects []*unstructured.Unstructured, out io.Writer) error {
@@ -112,7 +113,7 @@ func (c DiffCmd) Run(apiObjects []*unstructured.Unstructured, out io.Writer) err
 		}
 	}
 
-	if diffFound {
+	if diffFound && !c.NoErrorOnDiff {
 		return ErrDiffFound
 	}
 	return nil