Fix data race on err variable (#46)

Named return variable was potentially
mutated from multiple goroutines. Use
the library to do this safely.

go.mod

@@ -3,22 +3,21 @@ module github.com/argoproj/gitops-engine
 go 1.13
 
 require (
-	github.com/Masterminds/semver v1.5.0 // indirect
 	github.com/argoproj/pkg v0.0.0-20200102163130-2dd1f3f6b4de
 	github.com/evanphx/json-patch v4.2.0+incompatible
 	github.com/ghodss/yaml v1.0.0
 	github.com/google/shlex v0.0.0-20191202100458-e7afc7fbc510
 	github.com/grpc-ecosystem/grpc-gateway v1.3.1
-	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51 // indirect
-	github.com/patrickmn/go-cache v2.1.0+incompatible // indirect
+	github.com/k0kubun/colorstring v0.0.0-20150214042306-9440f1994b88 // indirect
 	github.com/pkg/errors v0.9.1
 	github.com/sergi/go-diff v1.1.0 // indirect
 	github.com/sirupsen/logrus v1.4.2
 	github.com/spf13/cobra v0.0.5
 	github.com/stretchr/testify v1.4.0
 	github.com/yudai/gojsondiff v1.0.1-0.20180504020246-0525c875b75c
 	github.com/yudai/golcs v0.0.0-20170316035057-ecda9a501e82 // indirect
-	gopkg.in/src-d/go-git.v4 v4.13.1 // indirect
+	github.com/yudai/pp v2.0.1+incompatible // indirect
+	golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e
 	gopkg.in/yaml.v2 v2.2.8 // indirect
 	k8s.io/api v0.16.6
 	k8s.io/apiextensions-apiserver v0.16.6