lint: Use TRAVIS_BRANCH in lint-git-commit-check.sh

test/lint/lint-git-commit-check.sh

@@ -23,10 +23,18 @@ while getopts "?" opt; do
   esac
 done
 
+# TRAVIS_BRANCH will be present in a Travis environment. For builds triggered
+# by a pull request this is the name of the branch targeted by the pull request.
+# https://docs.travis-ci.com/user/environment-variables/
+if [ -n "${TRAVIS_BRANCH}" ]; then
+  COMMIT_RANGE="$TRAVIS_BRANCH..HEAD"
+fi
+
 if [ -z "${COMMIT_RANGE}" ]; then
     if [ -n "$1" ]; then
       COMMIT_RANGE="HEAD~$1...HEAD"
     else
+      # This assumes that the target branch of the pull request will be master.
       MERGE_BASE=$(git merge-base HEAD master)
       COMMIT_RANGE="$MERGE_BASE..HEAD"
     fi