mktag: use default strbuf_read() hint

mktag: use default strbuf_read() hint

Change the hardcoded hint of 2^12 to 0. The default strbuf hint is
perfectly fine here, and the only reason we were hardcoding it is
because it survived migration from a pre-strbuf fixed-sized buffer.

See fd17f5b5f77 (Replace all read_fd use with strbuf_read, and get rid
of it., 2007-09-10) for that migration.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>