tools: add script to lint first PR commit message

Decouple first commit in pull request linting from Travis by using
the GitHub API to work out the first commit.

The shell script obtains the pull request number in one of the
following ways:
 1) supplied on the command line (use this to test against any PR)
 2) derived from the HEAD commit via the GitHub API

PR-URL: https://github.com/nodejs/node/pull/24030


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>