bump-deployments: refresh index before checking for changes

git diff-index reports stale stat cache entries as dirty after
Concourse's shallow clone resets file mtimes. Running
git update-index -q --refresh first syncs the stat cache so
diff-index gets an accurate read.

Author: neddp

ci/tasks/bump-deployments/task

@@ -29,6 +29,7 @@ function main() {
     git config user.email "cf-infrastructure@pivotal.io"
     git config user.name "cf-infra-bot"
 
+    git update-index -q --refresh
     git diff-index --quiet HEAD || git commit -am 'Update deployments'
 
     cp -r . ${ROOT}/bump-deployments-ci