fix set-e exit when checking api

paddle/scripts/paddle_build.sh

@@ -333,9 +333,8 @@ function assert_api_not_changed() {
     python ${PADDLE_ROOT}/tools/diff_api.py ${PADDLE_ROOT}/paddle/fluid/API.spec new.spec
     deactivate
 
-    API_CHANGE=`git diff --name-only HEAD^ | grep "paddle/fluid/API.spec"`
-    echo "checking API.spec change..."
-    echo "${GIT_PR_ID} , ${API_CHANGE}"
+    API_CHANGE=`git diff --name-only HEAD^ | grep "paddle/fluid/API.spec" | true`
+    echo "checking API.spec change, PR: ${GIT_PR_ID}, changes: ${API_CHANGE}"
     if [ ${API_CHANGE} ] && [ "${GIT_PR_ID}" != "" ]; then
         # TODO: curl -H 'Authorization: token ${TOKEN}'
         APPROVALS=`curl -H "Authorization: token ${GITHUB_API_TOKEN}" https://api.github.com/repos/PaddlePaddle/Paddle/pulls/${GIT_PR_ID}/reviews | \