write-or-die: fix the polarity of GIT_FLUSH environment variable

write-or-die: fix the polarity of GIT_FLUSH environment variable

When GIT_FLUSH is set to 1, true, on, yes, then we should disable
skip_stdout_flush, but the conversion somehow did the opposite.

With the understanding of the original motivation behind "skip" in
06f59e9f (Don't fflush(stdout) when it's not helpful, 2007-06-29),
we can sympathize with the current naming (we wanted to avoid
useless flushing of stdout by default, with an escape hatch to
always flush), but it is still not a good excuse.

Retire the "skip_stdout_flush" variable and replace it with "flush_stdout"
that tells if we do or do not want to run fflush().

Reported-by: Xiaoguang WANG <wxiaoguang@gmail.com>
Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>