tools: apply linting to first commit in PRs

Use Travis-CI to check the formatting of the first commit in a pull
request. This will hopefully reduce formatting errors and nits about
them in pull requests.

PR-URL: https://github.com/nodejs/node/pull/22452


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>