simplify realpath/bash_source logic

  http://stackoverflow.com/a/4774063/89484

Author: paulirish

diff-so-fancy

@@ -9,7 +9,8 @@ hash gsed 2> /dev/null && SED=gsed || SED=sed
 if hash diff-highlight 2> /dev/null; then
 	diff_highlight=diff-highlight
 else
-	diff_highlight="$( cd "$( dirname $(realpath "${BASH_SOURCE[0]}") )" && pwd )/third_party/diff-highlight/diff-highlight"
+	SCRIPT_PATH=$( cd $(dirname $0) ; pwd -P )
+	diff_highlight="$SCRIPT_PATH/third_party/diff-highlight/diff-highlight"
 fi
 
 color_code_regex="(\x1B\[([0-9]{1,3}(;[0-9]{1,3}){0,3})[m|K])?"