Fix trace for optional GitRepository.Spec.Reference

Check for existence of GitRepository.Spec.Reference when displaying a trace to
avoid error:

✗ template: tmpl:28:21: executing "tmpl" at <.GitRepository.Spec.Reference.Tag>: nil pointer evaluating *v1beta1.GitRepositoryRef.Tag

Fixes issue #1621
Manually tested using the use cases highlighted in the issue.

Signed-off-by: Allen Porter <allen@thebends.org>

cmd/flux/trace.go

@@ -191,6 +191,7 @@ Status:        Unknown
 GitRepository: {{.GitRepository.Name}}
 Namespace:     {{.GitRepository.Namespace}}
 URL:           {{.GitRepository.Spec.URL}}
+{{- if .GitRepository.Spec.Reference }}
 {{- if .GitRepository.Spec.Reference.Tag }}
 Tag:           {{.GitRepository.Spec.Reference.Tag}}
 {{- else if .GitRepository.Spec.Reference.SemVer }}
@@ -198,6 +199,7 @@ Tag:           {{.GitRepository.Spec.Reference.SemVer}}
 {{- else if .GitRepository.Spec.Reference.Branch }}
 Branch:        {{.GitRepository.Spec.Reference.Branch}}
 {{- end }}
+{{- end }}
 {{- if .GitRepository.Status.Artifact }}
 Revision:      {{.GitRepository.Status.Artifact.Revision}}
 {{- end }}