feat: --suppress-secrets of diff and apply commands (roboll#272)

Adds `--suppress-secrets` to `helmfile apply` and `helmfile diff`, so that the diff command omits the contents of secrets from its output. This is a security feature that should always be turned on for CI/CD use-cases.

With `--suppress-secrets`, the output when there is any change looks like:

```
Comparing bar stable/grafana
default, baz-grafana, Secret (v1) has changed:
+ Changes suppressed on sensitive content of type Secret
```

Resolves roboll#269

main.go

@@ -181,6 +181,10 @@ func main() {
 					Name:  "detailed-exitcode",
 					Usage: "return a non-zero exit code when there are changes",
 				},
+				cli.BoolFlag{
+					Name:  "suppress-secrets",
+					Usage: "suppress secrets in the output. highly recommended to specify on CI/CD use-cases",
+				},
 				cli.IntFlag{
 					Name:  "concurrency",
 					Value: 0,
@@ -189,7 +193,7 @@ func main() {
 			},
 			Action: func(c *cli.Context) error {
 				return findAndIterateOverDesiredStatesUsingFlags(c, func(state *state.HelmState, helm helmexec.Interface) []error {
-					return executeDiffCommand(c, state, helm, c.Bool("detailed-exitcode"))
+					return executeDiffCommand(c, state, helm, c.Bool("detailed-exitcode"), c.Bool("suppress-secrets"))
 				})
 			},
 		},
@@ -276,10 +280,14 @@ func main() {
 					Name:  "auto-approve",
 					Usage: "Skip interactive approval before applying",
 				},
+				cli.BoolFlag{
+					Name:  "suppress-secrets",
+					Usage: "suppress secrets in the diff output. highly recommended to specify on CI/CD use-cases",
+				},
 			},
 			Action: func(c *cli.Context) error {
 				return findAndIterateOverDesiredStatesUsingFlags(c, func(state *state.HelmState, helm helmexec.Interface) []error {
-					errs := executeDiffCommand(c, state, helm, true)
+					errs := executeDiffCommand(c, state, helm, true, c.Bool("suppress-secrets"))
 
 					// sync only when there are changes
 					if len(errs) > 0 {
@@ -441,7 +449,7 @@ func executeSyncCommand(c *cli.Context, state *state.HelmState, helm helmexec.In
 	return state.SyncReleases(helm, values, workers)
 }
 
-func executeDiffCommand(c *cli.Context, state *state.HelmState, helm helmexec.Interface, detailedExitCode bool) []error {
+func executeDiffCommand(c *cli.Context, state *state.HelmState, helm helmexec.Interface, detailedExitCode, suppressSecrets bool) []error {
 	args := args.GetArgs(c.String("args"), state)
 	if len(args) > 0 {
 		helm.SetExtraArgs(args...)
@@ -459,7 +467,7 @@ func executeDiffCommand(c *cli.Context, state *state.HelmState, helm helmexec.In
 	values := c.StringSlice("values")
 	workers := c.Int("concurrency")
 
-	return state.DiffReleases(helm, values, workers, detailedExitCode)
+	return state.DiffReleases(helm, values, workers, detailedExitCode, suppressSecrets)
 }
 
 func findAndIterateOverDesiredStatesUsingFlags(c *cli.Context, converge func(*state.HelmState, helmexec.Interface) []error) error {

state/state.go

@@ -215,7 +215,7 @@ func (state *HelmState) SyncReleases(helm helmexec.Interface, additionalValues [
 }
 
 // DiffReleases wrapper for executing helm diff on the releases
-func (state *HelmState) DiffReleases(helm helmexec.Interface, additionalValues []string, workerLimit int, detailedExitCode bool) []error {
+func (state *HelmState) DiffReleases(helm helmexec.Interface, additionalValues []string, workerLimit int, detailedExitCode, suppressSecrets bool) []error {
 	var wgRelease sync.WaitGroup
 	var wgError sync.WaitGroup
 	errs := []error{}
@@ -256,6 +256,10 @@ func (state *HelmState) DiffReleases(helm helmexec.Interface, additionalValues [
 					flags = append(flags, "--detailed-exitcode")
 				}
 
+				if suppressSecrets {
+					flags = append(flags, "--suppress-secrets")
+				}
+
 				if len(errs) == 0 {
 					if err := helm.DiffRelease(release.Name, normalizeChart(state.basePath, release.Chart), flags...); err != nil {
 						errs = append(errs, err)