Use category to distinguish between changes

[kszucs]

ChangeSource.category
ChangeFilter.category_fn
Builder.category

Github hook creates 3 kinds of changes, distinguishable by their category field:

    None: This change is a push to a branch.

        Use util.ChangeFilter(category=None, repository="http://github.com/<org>/<project>")

    'tag': This change is a push to a tag.

        Use util.ChangeFilter(category='tag', repository="http://github.com/<org>/<project>")

    'pull': This change is from a pull-request creation or update.

        Use util.ChangeFilter(category='pull', repository="http://github.com/<org>/<project>") In this case, the GitHub step must be used instead of the standard Git in order to be able to pull GitHub’s magic refs. With this method, the GitHub step will always checkout the branch merged with latest master. This allows to test the result of the merge instead of just the source branch. Note that you can use the GitHub for all categories of event.

[kszucs]

resolved via 4176633