Travis linter false-negatives for commit message checks

[mmarchini]

Saw this on two PRs today (#23512 and #22079), Travis is complaining that "subsystem is missing". In both cases commit message is following our guidelines and have a valid subsystem in their titles.

[richardlau]

#23725

[refack]

Opened #23739 to stop linting...

[richardlau]

The original issue is that Travis was misidentifying the commits for a PR.

Rather than rely on Travis to work out which commits should be linted, #24030, #24076, #24093 changed the Travis linter to use GitHub's API to query for commits for the PR being tested so should now be consistent with the list of commits for a PR in the GitHub web UI.