tools: add script to lint first PR commit message

[richardlau]

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. TRAVIS_PULL_REQUEST if set and != "false"
3. derived from the HEAD commit via the GitHub API

The GitHub API should be more consistent with the web UI in terms of finding
the first commit for a pull request.

This is an alternative to #23758 with the bonus that
you can run the script against any pull request number.

Checklist

• commit message follows commit guidelines

[nodejs-github-bot]

@richardlau build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/1455/pipeline

[richardlau]

Examples:

Run locally, no pull request specified (script works out based on current HEAD commit):

-bash-4.2$ bash tools/lint-pr-commit-message.sh
Linting the first commit message for pull request 24030
according to the guidelines at https://goo.gl/p2fr5Q.
Commit message for 826115ee852b8aef3dd6e9578c8b0190605c0e98 is:
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) TRAVIS_PULL_REQUEST if set and != "false"
 3) derived from the HEAD commit via the GitHub API
  ✔  826115ee852b8aef3dd6e9578c8b0190605c0e98
     ✔  0:0      skipping fixes-url                        fixes-url
     ✔  0:0      blank line after title                    line-after-title
     ✔  0:0      line-lengths are valid                    line-length
     ✔  0:0      valid subsystems                          subsystem
     ✔  0:0      Title is formatted correctly.             title-format
     ✔  0:0      Title is <= 50 columns.                   title-length
-bash-4.2$

Checking a completely different PR:

-bash-4.2$ bash tools/lint-pr-commit-message.sh 23758
Linting the first commit message for pull request 23758
according to the guidelines at https://goo.gl/p2fr5Q.
Commit message for fa99a11b207932c6d02ccfc9e8308984ed0f1f76 is:
tools: add script to lint commit messages

This script attempts to guess the target upstream branch.
To manually specify, e.g. to target `canary-base`:
  TARGET_BRANCH=canary-base bash lint-commit-message.sh
  ✔  fa99a11b207932c6d02ccfc9e8308984ed0f1f76
     ✔  0:0      skipping fixes-url                        fixes-url
     ✔  0:0      blank line after title                    line-after-title
     ✔  0:0      line-lengths are valid                    line-length
     ✔  0:0      valid subsystems                          subsystem
     ✔  0:0      Title is formatted correctly.             title-format
     ✔  0:0      Title is <= 50 columns.                   title-length
-bash-4.2$

On Travis (for this PR), https://travis-ci.com/nodejs/node/jobs/155889224:

[richardlau]

Moved the Travis specific stuff out of the script back into .travis.yml.

[richardlau]

Longer term we could teach core-validate-commit:

1. to take a PR argument and lint all the commits for that PR (as determined by GitHub's API)
2. to ignore "fixup!" and "squash!" commit messages

and then replace the shell script here with a direct call to npx -q core-validate-commit ....

[Trott]

Lite CI: https://ci.nodejs.org/job/node-test-pull-request-lite-pipeline/1482/

[Trott]

Landed in 7489ee8

[addaleax]

@Trott @richardlau This seems to have broken Travis CI for non-PR branches without TRAVIS_PULL_REQUEST?