nfv?asprintf are broken without va_copy, workaround them.

nfv?asprintf are broken without va_copy, workaround them.

* drop nfasprintf.
* move nfvasprintf into imap-send.c back, and let it work on a 8k buffer,
  and die() in case of overflow. It should be enough for imap commands, if
  someone cares about imap-send, he's welcomed to fix it properly.
* replace nfvasprintf use in merge-recursive with a copy of the strbuf_addf
  logic, it's one place, we'll live with it.
  To ease the change, output_buffer string list is replaced with a strbuf ;)
* rework trace.c to call vsnprintf itself.  It's used to format strerror()s
  and git command names, it should never be more than a few octets long, let
  it work on a 8k static buffer with vsnprintf or die loudly.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>