git-review: add -q to git branch --set-upstream-to command

Git is currently spamming me with:

$ git change upstreamq
Branch upstreamq set up to track remote branch master from origin.
git-review: created branch upstreamq tracking origin/master.
$

I introduced this by submitting my other change earlier this morning.

TBR=adg

Change-Id: I68a96d68541d211bda546ed490085632982bc905
Reviewed-on: https://go-review.googlesource.com/1430
Reviewed-by: Russ Cox <rsc@golang.org>