When the command line has "--" disambiguator, we take the remainder of
argv[] as "prune_data", but when --stdin is given at the same time,
we need to append to the existing prune_data and end up attempting to
realloc(3) it.  That would not work.

Fix it by consistently using append_prune_data() throughout the input
processing.  Also avoid counting the number of existing paths in the
function over and over again.

Signed-off-by: Junio C Hamano <gitster@pobox.com>