do not recalculate some vars if the $PWD hasn't changed

vars that will not be recalculated whithout path change :
* LP_VCS_TYPE : the type of repository we're in (git / svn / nothing
* / ...)
* LP_PWD : the (maybe shortened) path
* LP_MARK : the prompt mark which depend on the type of repository
* we're in
* LP_PERM : the color of the colon indicating the permission for the
* current directory