zsh completion: discard progress from internal bazel invocations
zsh completion: discard progress from internal bazel invocations

When computing completion options, bazel has to be called. For those
calls, discard the progress output (on stderr) to avoid them messing
up the terminial. Fixes #2279.

Change-Id: Ic1536058a44dac687ad59042ec56af50a4125f3b
PiperOrigin-RevId: 160525601

cushon/bazel,aehlig/bazel,twitter-forks/bazel,meteorcloudy/bazel,spxtr/bazel,variac/bazel,snnn/bazel,dslomov/bazel-windows,twitter-forks/bazel,meteorcloudy/bazel,davidzchen/bazel,spxtr/bazel,werkt/bazel,dslomov/bazel-windows,spxtr/bazel,ButterflyNetwork/bazel,davidzchen/bazel,katre/bazel,damienmg/bazel,katre/bazel,perezd/bazel,bazelbuild/bazel,ulfjack/bazel,variac/bazel,dslomov/bazel,damienmg/bazel,perezd/bazel,spxtr/bazel,meteorcloudy/bazel,werkt/bazel,aehlig/bazel,werkt/bazel,dslomov/bazel,variac/bazel,damienmg/bazel,dslomov/bazel,dropbox/bazel,dslomov/bazel-windows,davidzchen/bazel,aehlig/bazel,cushon/bazel,davidzchen/bazel,perezd/bazel,damienmg/bazel,ButterflyNetwork/bazel,spxtr/bazel,dslomov/bazel,dropbox/bazel,dslomov/bazel,werkt/bazel,bazelbuild/bazel,dslomov/bazel-windows,perezd/bazel,perezd/bazel,werkt/bazel,akira-baruah/bazel,dropbox/bazel,cushon/bazel,safarmer/bazel,snnn/bazel,damienmg/bazel,ulfjack/bazel,katre/bazel,ButterflyNetwork/bazel,twitter-forks/bazel,katre/bazel,dslomov/bazel-windows,meteorcloudy/bazel,dropbox/bazel,ulfjack/bazel,ulfjack/bazel,werkt/bazel,meteorcloudy/bazel,snnn/bazel,bazelbuild/bazel,twitter-forks/bazel,perezd/bazel,meteorcloudy/bazel,dropbox/bazel,aehlig/bazel,bazelbuild/bazel,cushon/bazel,snnn/bazel,variac/bazel,akira-baruah/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,akira-baruah/bazel,katre/bazel,variac/bazel,safarmer/bazel,davidzchen/bazel,ulfjack/bazel,variac/bazel,ButterflyNetwork/bazel,ulfjack/bazel,cushon/bazel,snnn/bazel,spxtr/bazel,snnn/bazel,dslomov/bazel,safarmer/bazel,aehlig/bazel,davidzchen/bazel,cushon/bazel,aehlig/bazel,bazelbuild/bazel,safarmer/bazel,dslomov/bazel-windows,akira-baruah/bazel,safarmer/bazel,twitter-forks/bazel,katre/bazel,snnn/bazel,twitter-forks/bazel,davidzchen/bazel,ButterflyNetwork/bazel,bazelbuild/bazel,spxtr/bazel,perezd/bazel,damienmg/bazel,damienmg/bazel,ulfjack/bazel,dropbox/bazel,aehlig/bazel,akira-baruah/bazel,variac/bazel,twitter-forks/bazel,dslomov/bazel,safarmer/bazel,akira-baruah/bazel