If f_name_cmp() discovers that two directory strings compare to an

If f_name_cmp() discovers that two directory strings compare to an
equal value without being equal pointers, substitute one of the
pointers for the other in the file list.  This optimizes future name
comparisons.  Note also that this optimization won't be triggered
very often (because rsync tends to send the names grouped by dir-
name at transmission time), but it's nice to be able to assume that
all files in the same dir have identical dir-name pointers after the
qsort is finished.