Fix NPE during direct push to branch closing a change

Direct pushing into a branch and closing changes which are still open
by matching commit SHA-1s against open change patch sets resulted in
the following stack trace:

2009-08-24 12:01:05,064::ERROR: com.google.gerrit.server.ssh.BaseCommand  - Internal server error (user foo account N) during git-receive-pack '/foo.git'
java.lang.NullPointerException
        at com.google.gerrit.client.reviewdb.Change.setCurrentPatchSet(Change.java:423)
        at com.google.gerrit.server.ssh.commands.Receive.markChangeMergedByPush(Receive.java:1353)

This was caused by the PatchSetInfo object not being loaded into memory
before we tried to update the open change with the final commit data.

Change-Id: Ic6a9671820916463688e605a8f27c21bf76627d9
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed