vreportf: avoid intermediate buffer

When we call "die(fmt, args...)", we end up in vreportf with
two pieces of information:

  1. The prefix "fatal: "

  2. The original fmt and va_list of args.

We format item (2) into a temporary buffer, and then fprintf
the prefix and the temporary buffer, along with a newline.
This has the unfortunate side effect of truncating any error
messages that are longer than 4096 bytes.

Instead, let's use separate calls for the prefix and
newline, letting us hand the item (2) directly to vfprintf.
This is essentially undoing d048a96 (print
warning/error/fatal messages in one shot, 2007-11-09), which
tried to have the whole output end up in a single `write`
call.

But we can address this instead by explicitly requesting
line-buffering for the output handle, and by making sure
that the buffer is empty before we start (so that outputting
the prefix does not cause a flush due to hitting the buffer
limit).

We may still break the output into two writes if the content
is larger than our buffer, but there's not much we can do
there; depending on the stdio implementation, that might
have happened even with a single fprintf call.

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

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