Stop multipe changelog generators from crashing into each other

.build/generate_changelog.sh

@@ -48,6 +48,15 @@ fi
 git config --global user.email "jrnl.bot@gmail.com"
 git config --global user.name "Jrnl Bot"
 git checkout $BRANCH
+git pull origin $BRANCH
+
+# Check if branch has been updated since this build started
+# This tends to happen if multiple things have been merged in at the same time.
+if [[ $(git rev-parse head) != $TRAVIS_COMMIT ]]; then
+  echo "${BRANCH} has been updated since build started. Aborting changelog."
+  exit 0
+fi
+
 git add "$FILENAME"
 git commit -m "Updating changelog [ci skip]"
 git push https://${GITHUB_TOKEN}@github.com/jrnl-org/jrnl.git $BRANCH