Simplify the GitReferenceUpdatedListener.Event API

So far the Event exposed the getUpdates method which implied that one
Event could contain updates of more than one reference. However, we
never made use of that feature. The only implementation of the Event
always returns only one Update.

The assumption that one Event could contain more than one reference
Update also made coding in [1] more complex than necessary.

This change simplifies that API in the sense that one Event corresponds
to one ref update only.

[1] https://gerrit-review.googlesource.com/#/c/50351/2/src/main/java/com/googlesource/gerrit/plugins/replication/PushResultProcessing.java

Change-Id: Ifdb27e22ae83f30a6af7f0b4e6bf6acbd85fe6e4
3 files changed