PathFilter::setFilter: don't copy items before overwriting them

Average BenchQuickOpen::benchProjectFileFilter_setFilter results before
and at this commit in milliseconds per iteration:
Data row    Before  At
 1000-      0.0018  0.0018
 1000-bar   0.55    0.50
 1000-  1   0.57    0.49
 1000-f/b   0.47    0.41
10000-      0.0018  0.0018
10000-bar   5.8     5.3
10000-  1   5.5     4.7
10000-f/b   5.0     4.4