add: free leaked pathspec after add_files_to_cache()

add: free leaked pathspec after add_files_to_cache()

After run_diff_files, we throw away the rev_info struct,
including the pathspec that we copied into it, leaking the
memory. this is probably not a big deal in practice. We
usually only run this once per process, and the leak is
proportional to the pathspec list we're already holding in
memory.

But it's still a leak, and it pollutes leak-checker output,
making it harder to find important leaks.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>