Send new patchset event after its available

gerrit stream-events sent notification of a new patch set before it
created the reference in the Git repository. This confused some CI
systems that were able to read the stream-events message and look for
the reference before the server was able to create it.

Bug: issue 755
Change-Id: I14eb9d19984701eb9afbb13e006a24057e8c3473
Signed-off-by: Shawn O. Pearce <sop@google.com>