This patch calls the Publishers associated with a matrix build after completing the parent build.

This is more of an RFC than a patch because it may be that certain publishers do not correctly handle being called from the parent matrix build. Indeed this patch was developed to solve a specific issue in our environment and further development may be needed to make this more generic.

Specifically, this was needed in order to support the Git plugin's "Push Merges back to origin" feature in the context of matrix builds as this is implemented using a publisher. Without this patch the publisher could only run at the conclusion of every child build which is not very useful for publishers which need to know the result of the parent (e.g. the git plugin should only push if all child builds succeeded).