Fill in TRAVIS_COMMIT_RANGE if missing

Workaround for travis-ci/travis-ci#1719

script/test

@@ -6,6 +6,11 @@ set -e
 STATUS=0
 bats -t test || STATUS="$?"
 
+if [ "$TRAVIS_PULL_REQUEST" != "false" ] && [ -z "$TRAVIS_COMMIT_RANGE" ]; then
+  first_sha="$(curl -fsSL https://github.com/${TRAVIS_REPO_SLUG}/pull/${TRAVIS_PULL_REQUEST}.patch | head -1 | awk '{print $2}')"
+  export TRAVIS_COMMIT_RANGE="${first_sha}^..${TRAVIS_COMMIT}"
+fi
+
 if [ "$TRAVIS_SECURE_ENV_VARS" = "true" ]; then
   ./script/mirror update "$TRAVIS_COMMIT_RANGE"
 elif [ "$TRAVIS_PULL_REQUEST" != "false" ]; then