Work around git safe.directory enforcement

Git wants to ensure that you do not read .git owned by other users.
But we fetch+build as 'build' user, and run tests as root user.  Those
tests calculate git topdir using git rev-parse --show-toplevel, which
git now fails.

Setting safe.directory, seems wrong.  Let's just use bash to figure
out the top dir.

tests/common/config.sh

@@ -2,7 +2,14 @@
 
 set -e
 
-build_path=$(git rev-parse --show-toplevel)
+build_path=$(pwd)
+while [ "${build_path}" != "/" -a ! -d "${build_path}/.git" ]; do
+	build_path=$(dirname ${build_path})
+done
+if [ ! -d "${build_path}/.git" ]; then
+	echo "Not inside git directory" 1>&2
+	exit 1
+fi
 
 # Save the configuration files in tmp.
 save_config ()