ident: use a dynamic strbuf in fmt_ident

ident: use a dynamic strbuf in fmt_ident

Now that we accept arbitrary-sized names and email
addresses, the only remaining limit is in the actual
formatting of the names into a buffer. The current limit is
1000 characters, which is not likely to be reached, but
using a strbuf is one less error condition we have to worry
about.

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