ref-filter: drop sprintf and strcpy calls

ref-filter: drop sprintf and strcpy calls

The ref-filter code comes from for-each-ref, and inherited a
number of raw sprintf and strcpy calls. These are generally
all safe, as we custom-size the buffers, or are formatting
numbers into sufficiently large buffers. But we can make the
resulting code even simpler and more obviously correct by
using some of our helper functions.

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