Update prepare.sh

third_party/prepare.sh

 #!/bin/bash -e
 
+git_version=$(git --version)
+if [ >= "1.8.4" ]; then
+    echo "Since git 1.8.4 (August 2013), you don't have to be at top-level to run git submodule update."
+else
+    echo "You have to update your git version to 1.8.4 or later."
+    exit -1
+fi
+
 cd $(dirname $0)
 
 git submodule sync