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>

write-or-die.c

@@ -18,20 +18,20 @@
  */
 void maybe_flush_or_die(FILE *f, const char *desc)
 {
-	static int skip_stdout_flush = -1;
-
 	if (f == stdout) {
-		if (skip_stdout_flush < 0) {
-			skip_stdout_flush = git_env_bool("GIT_FLUSH", -1);
-			if (skip_stdout_flush < 0) {
+		static int force_flush_stdout = -1;
+
+		if (force_flush_stdout < 0) {
+			force_flush_stdout = git_env_bool("GIT_FLUSH", -1);
+			if (force_flush_stdout < 0) {
 				struct stat st;
 				if (fstat(fileno(stdout), &st))
-					skip_stdout_flush = 0;
+					force_flush_stdout = 1;
 				else
-					skip_stdout_flush = S_ISREG(st.st_mode);
+					force_flush_stdout = !S_ISREG(st.st_mode);
 			}
 		}
-		if (skip_stdout_flush && !ferror(f))
+		if (!force_flush_stdout && !ferror(f))
 			return;
 	}
 	if (fflush(f)) {