Also send commit_messages field for PR merge push events [SSW-2040]

[erosdome]

New Pull Request Checklist

• Run go fmt on your files (e.g. go fmt ./service/common.go, or on the whole service folder: go fmt ./service/...)
• Write tests for your code
  • The tests should cover both "success" and "error" cases.
  • The tests should also check all the returned variables, don't ignore any returned value!
  • Ideally the tests should be easily readable, we usually use tests to document our code instead of code comments.
    An example, if you'd write a comment like "Given X this function will return Y" or
    "Beware, if the input is X this function will return Y" then you should implement this as
    a unit test, instead of writing it as a comment.
• If your Pull Request is more than a bug fix you should also check README.md and change/add the descriptions there - also
  feel free to add yourself as a contributor if you implement support for a new service ;)
• Before creating the Pull Request you should also run bitrise run test with the Bitrise CLI,
  to perform all the automatic checks (which will run on your Pull Request when you open it).

Summary of Pull Request

This Pull Request makes the code 10x faster, while reducing
memory consumption by 99%, as well as it implements 5 new service support ...
Something like this ;)

[molnarm]

Looks good to me IF the comment about the PR merge is still true (only this event is sent when a PR is merged instead of a proper one with details of the merge commit). If not, this could cause duplicate builds.