Set CI_BRANCH_BASE for bundlewatch on push events

[tomalec]

Changes proposed in this Pull Request:

Set BRANCH_BASE for bundlewatch on push events,
workaround bundlewatch/bundlewatch#423.

Previously if no base was given, it was using hardcoded master which does not exist in our repo.
See https://app.travis-ci.com/github/woocommerce/google-listings-and-ads/jobs/522666112#L14735

[ERROR] Unable to fetch fileDetails for baseBranch=master from https://service.bundlewatch.io/store code=Request failed with status code 404
[INFO] master !== trunk, no results saved
[INFO] Saving results

Screenshots:

Detailed test instructions:

1. Run buildwatch with those changes on push event - with our current setup, this means run them in trunk :|

Changelog entry

[ecgan]

Putting some additional notes here after discussion with @tomalec . Bundlewatch uses ci-env package. When the Travis event type is a push event, the pull_request_target_branch in ci-env will be an empty string - see https://github.com/siddharthkp/ci-env/blob/bccc71df40b351d7aa0bd4680fec566302c9bbcf/index.js#L28-L35.

pull_request_target_branch is used by bundlewatch in https://github.com/bundlewatch/bundlewatch/blob/98b7b789d40032b301351d2c0b02a1c443db2b7a/src/app/config/getCIVars.js#L10. Because of the empty string in push event, bundlewatch will use master as the repoBranchBase. See https://github.com/bundlewatch/bundlewatch/blob/98b7b789d40032b301351d2c0b02a1c443db2b7a/src/app/config/getConfig.js#L17. Our repo does not have master branch, which is why this can be a problem in push events.

Push events can happen when we merge a pull request from a feature branch into trunk branch - a commit is pushed onto trunk, triggering build.

Specifying the CI_BRANCH_BASE should work as it take precedence over pull_request_target_branch provided by ci-env. See https://github.com/bundlewatch/bundlewatch/blob/98b7b789d40032b301351d2c0b02a1c443db2b7a/src/app/config/getCIVars.js#L10.

[ecgan]

LGTM. I think we can go ahead and try this out. 🚀

[tomalec]

🎉 trunk successfully saved
https://travis-ci.com/github/woocommerce/google-listings-and-ads/jobs/522969367#L14739-L14742

[INFO] Retrieving comparison
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