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

scripts/zsh_completion/_bazel

diff --git a/scripts/zsh_completion/_bazel b/scripts/zsh_completion/_bazel
index 5ae38d3..5958830 100644
--- a/scripts/zsh_completion/_bazel
+++ b/scripts/zsh_completion/_bazel
@@ -40,7 +40,7 @@
 
 : ${BAZEL_COMPLETION_PACKAGE_PATH:=%workspace%}
 : ${BAZEL:=bazel}
-b() { ${BAZEL} --noblock_for_lock "$@"; }
+b() { ${BAZEL} --noblock_for_lock "$@" 2>/dev/null; }
 
 # Default cache lifetime is 1 week
 zstyle -s ":completion:${curcontext}:" cache-lifetime lifetime