Fix Gerrit sha1 calculation

Change-Id: I962ce21cba85e8bf3f426241d2053b83835cd55c
diff --git a/common.env b/common.env
index 617e06d..324a478 100644
--- a/common.env
+++ b/common.env
@@ -14,5 +14,5 @@
 
 # Image version
 IMAGE_DIR=$(dir $(realpath $(lastword $(MAKEFILE_LIST))))gerrit
-HEAD_SHA1=$(shell git $(IMAGE_DIR) -type f -exec cat {} \; | shasum | cut -c 1-20)
+HEAD_SHA1=$(shell find $(IMAGE_DIR) -type f -exec cat {} \; | shasum | cut -c 1-20)
 IMAGE_TAG=$(GERRIT_VERSION).$(GERRIT_PATCH)-$(HEAD_SHA1)