Compatibility fix for Design Assets Management

design.sh uses `readlink -m` to determine canonical path of git dir,
which is not available on all *nix systems (including Darwin).

Instead, use `pwd -P` to get canonical directory path in a more
cross-platform compatible way.

This is a change to an actual script rather than just a test, and it
should make the Design Assets Management functionality of scm_breeze
now function properly on MacOSX.

lib/design.sh

@@ -23,7 +23,7 @@
 
 # Add ignore rule to .git/info/exclude if not already present
 _design_add_git_exclude(){
-  local git_dir="$(cd $1 && readlink -m $(git rev-parse --git-dir))"
+  local git_dir="$(cd $1 && cd `git rev-parse --git-dir` && pwd -P)"
   if [ -e "$git_dir/info/exclude" ] && ! $(grep -q "$project_design_dir" "$git_dir/info/exclude"); then
     echo "$project_design_dir" >> "$git_dir/info/exclude"
   fi