Ensure formatter doesn't change anything in pr (#8)

* Ensure formatter doesn't change anything in pr

* Use [ instead of [[

* Fix quotes

* Add mage target

* Fix format error

.buildkite/pipeline.yml

@@ -2,6 +2,7 @@ steps:
   - commands:
     - go get github.com/magefile/mage
     - mage -v
+    - mage -v gitdirty
     plugins:
       - docker#v3.2.0:
           image: "golang:1.12"

magefile.go

@@ -3,6 +3,7 @@
 package main
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -74,6 +75,17 @@ func Deps() error {
 	return nil
 }
 
+// Fails if the repo is dirty
+func GitDirty() error {
+	o, err := sh.Output("git", "status", "--porcelain")
+	if o != "" || err != nil {
+		// Show the full status
+		sh.Run("git", "status")
+		return fmt.Errorf("git is dirty")
+	}
+	return nil
+}
+
 func parseGoModule() (string, error) {
 	modContents, err := ioutil.ReadFile("go.mod")
 	if err != nil {