ls-files: pass prefix length explicitly to prune_cache()

ls-files: pass prefix length explicitly to prune_cache()

The function prune_cache() relies on the fact that it is only called on
max_prefix and sneakily uses the matching global variable max_prefix_len
directly.  Tighten its interface by passing both the string and its
length as parameters.  While at it move the NULL check into the function
to collect all cache-pruning related logic in one place.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Reviewed-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>