Add work path CLI option (#6922)

Makes it possible to set the work path as a CLI option instead of
relying on environment variables which are somewhat opaque

contrib/pr/checkout.go

@@ -43,7 +43,7 @@ func runPR() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	setting.SetCustomPathAndConf("", "")
+	setting.SetCustomPathAndConf("", "", "")
 	setting.NewContext()
 
 	setting.RepoRootPath, err = ioutil.TempDir(os.TempDir(), "repos")