environ: GIT_FLUSH should be made a usual Boolean

This uses atoi() and checks if the result is not zero to decide what
to do.  Turning it into the usual Boolean environment variable to
use git_env_bool() would not break those who have been using "set to
0, or set to non-zero, that can be parsed with atoi()" values, but
will match the expectation of those who expected "true" to mean
"yes".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -705,6 +705,7 @@ for further details.
 	waiting for someone with sufficient permissions to fix it.
 
 `GIT_FLUSH`::
+// NEEDSWORK: make it into a usual Boolean environment variable
 	If this environment variable is set to "1", then commands such
 	as 'git blame' (in incremental mode), 'git rev-list', 'git log',
 	'git check-attr' and 'git check-ignore' will

write-or-die.c

@@ -23,6 +23,7 @@ void maybe_flush_or_die(FILE *f, const char *desc)
 
 	if (f == stdout) {
 		if (skip_stdout_flush < 0) {
+			/* NEEDSWORK: make this a normal Boolean */
 			cp = getenv("GIT_FLUSH");
 			if (cp)
 				skip_stdout_flush = (atoi(cp) == 0);