[Triage Metadata] Improve title/description of generated PRs

[stephenmcgruer]

It would be really awesome if the title or description of generated PRs could include the changes made in some form. For example, taking https://github.com/web-platform-tests/wpt-metadata/pull/134 as an example, it would be cool if it looked something like:

Triage link-multiple-error-events.html on chrome This PR was created for metadata triaged through the wpt.fyi /api/metadata/triage endpoint. It triages the following files:

• html/semantics/document-metadata/the-link-element/link-multiple-error-events.html (chrome, https://crbug.com/922618)

See the documentation for more information about how to use this service.

Doing all of that would maybe be a bit much, but we could at least make the PR titles (and the underlying commit titles) a bit better without much work I think. If multiple tests were triaged we can always fall back to something generic (at which point the description becomes more useful):

Triage X tests via wpt.fyi