Log the incoming git refs events for replication

Display on the pull_replication_log all the git-ref event
details for the events received and selected by the pull-replication
plugin for being fired to the remotes.

Preparatory work for helping with the troubleshooting of
errors in processing the incoming events.

Bug: Issue 15608
Change-Id: If9cf8041ed261d42f849e8e80c14266ceae70e68