feat(templater) Add a template function to insert surrounding repo's Git hash

A template function has been added that allows one to template the
Git hash of the surrounding repo. This is useful to be able to inspect the
deployment revision of an object in Kubernetes.

templater/templater.go

@@ -15,6 +15,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path"
 	"strings"
 	"text/template"
@@ -169,6 +170,14 @@ func templateFuncs(rs *context.ResourceSet) template.FuncMap {
 		return string(b)
 	}
 	m["passLookup"] = GetFromPass
+	m["gitHEAD"] = func() (string, error) {
+			out, err := exec.Command("sh", "-c", "git rev-parse HEAD").Output()
+			if err != nil {
+				return "", err
+			}
+			output := strings.TrimSpace(string(out))
+			return output, nil
+		}
 	m["lookupIPAddr"] = GetIPsFromDNS
 	m["insertFile"] = func(file string) (string, error) {
 		data, err := ioutil.ReadFile(path.Join(rs.Path, file))