protobuf benchs: invoke protoc once with all protos (#623)

Before:

    $ time ./compile-protos.sh
    ...
    ./compile-protos.sh  102.82s user 8.70s system 153% cpu 1:12.75 total

After:

    $ time ./compile-protos.sh
    ...
    ./compile-protos.sh  6.26s user 0.57s system 141% cpu 4.820 total
diff --git a/protobuf/benchmarks/compile-protos.sh b/protobuf/benchmarks/compile-protos.sh
index b01cb47..8612662 100755
--- a/protobuf/benchmarks/compile-protos.sh
+++ b/protobuf/benchmarks/compile-protos.sh
@@ -30,8 +30,6 @@
 
 mkdir -p ${OUTPUT_DIR}
 
-for proto in ${PROTOS[@]}
-do
-  echo "PROTOC ${proto}"
-  protoc -I"${SCRIPT_DIR}" --dart_out="${OUTPUT_DIR}" "${SCRIPT_DIR}/$proto"
-done
+set -x
+
+protoc -I"${SCRIPT_DIR}" --dart_out="${OUTPUT_DIR}" "${PROTOS[@]/#/$SCRIPT_DIR/}"