silence a bunch of format-zero-length warnings

silence a bunch of format-zero-length warnings

This can be observed in many versions of gcc and still exists with 4.9.0:

  wt-status.c: In function ‘wt_status_print_unmerged_header’:
  wt-status.c:191:2: warning: zero-length gnu_printf format string [-Wformat-zero-length]
    status_printf_ln(s, c, "");
    ^

The user have long been told to pass -Wno-format-zero-length, but a
patch that avoids warning altogether is not too noisy, so let's do
so.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>