build: fetch target branch in Kokoro to resolve diff error

Author: meltsufin

.kokoro/common.sh

@@ -236,6 +236,7 @@ function generate_modified_modules_list() {
   # grep returns 1 (error code) and exits the pipeline if there is no match
   # If there is no match, it will return true so the rest of the commands can run
   git config --global --add safe.directory $(realpath .)
+  git fetch origin "${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}" || true
   modified_files=$(git diff --name-only "${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}...${KOKORO_GITHUB_PULL_REQUEST_COMMIT}")
   printf "Modified files:\n%s\n" "${modified_files}"