rev-list: handle %x00 NUL in user format

rev-list: handle %x00 NUL in user format

The code paths for showing commits in "git log" and "git
rev-list --graph" correctly handle embedded NULs by looking
only at the resulting strbuf's length, and never treating it
as a C string. The code path for regular rev-list, however,
used printf("%s"), which resulted in truncated output. This
patch uses fwrite instead, like the --graph code path.

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