fix report concurrency crash when helm-diff used as a library (#256)

the Manifests codepath was calling setupReportFormat on a mutable global map,
which means that calling Manifests from different gothreads is an error,
and can produce a panic with this (abbreviated) traceback:

fatal error: concurrent map writes

goroutine 668 [running]:
runtime.throw(0x27cb973, 0x15)
 /usr/local/go/src/runtime/panic.go:1116 +0x72 fp=0xc003a8c710 sp=0xc003a8c6e0 pc=0x437892
runtime.mapassign_faststr(0x23fc3a0, 0xc019650120, 0x27a837d, 0x6, 0xc00012f408)
 /usr/local/go/src/runtime/map_faststr.go:211 +0x3f1 fp=0xc003a8c778 sp=0xc003a8c710 pc=0x414811
github.com/databus23/helm-diff/v3/diff.setupDiffReport(...)
 /go/pkg/mod/github.com/databus23/helm-diff/[email protected]/diff/report.go:99
github.com/databus23/helm-diff/v3/diff.(*Report).setupReportFormat(0x3dafe80, 0x27a5ada, 0x4)
 /go/pkg/mod/github.com/databus23/helm-diff/[email protected]/diff/report.go:67 +0x111 fp=0xc003a8c7b0 sp=0xc003a8c778 pc=0x20e7291
github.com/databus23/helm-diff/v3/diff.Manifests(0xc009ffaf60, 0xc009ffb3e0, 0xc0061f73e0, 0x1, 0x1, 0xc0061f7300, 0x5, 0x27a5ada, 0x4, 0x2b26460, ...)

Author: novas0x2a

diff/diff.go

@@ -20,6 +20,7 @@ import (
 
 // Manifests diff on manifests
 func Manifests(oldIndex, newIndex map[string]*manifest.MappingResult, suppressedKinds []string, showSecrets bool, context int, output string, stripTrailingCR bool, to io.Writer) bool {
+	report := Report{}
 	report.setupReportFormat(output)
 	seenAnyChanges := false
 	emptyMapping := &manifest.MappingResult{}

diff/report.go

@@ -52,19 +52,17 @@ type ReportTemplateSpec struct {
 	Change    string
 }
 
-var report Report
-
 // setupReportFormat: process output argument.
 func (r *Report) setupReportFormat(format string) {
 	switch format {
 	case "simple":
-		setupSimpleReport(&report)
+		setupSimpleReport(r)
 	case "template":
-		setupTemplateReport(&report)
+		setupTemplateReport(r)
 	case "json":
-		setupJSONReport(&report)
+		setupJSONReport(r)
 	default:
-		setupDiffReport(&report)
+		setupDiffReport(r)
 	}
 }
 
@@ -126,7 +124,7 @@ func printSimpleReport(r *Report, to io.Writer) {
 		"MODIFY": 0,
 	}
 	for _, entry := range r.entries {
-		fmt.Fprintf(to, ansi.Color("%s %s", report.format.changestyles[entry.changeType].color)+"\n",
+		fmt.Fprintf(to, ansi.Color("%s %s", r.format.changestyles[entry.changeType].color)+"\n",
 			entry.key,
 			r.format.changestyles[entry.changeType].message,
 		)