write-or-die: make GIT_FLUSH a Boolean environment variable

Among Git's environment variable, the ones marked as "Boolean"
accept values in a way similar to Boolean configuration variables,
i.e. values like 'yes', 'on', 'true' and positive numbers are
taken as "on" and values like 'no', 'off', 'false' are taken as
"off".
GIT_FLUSH can be used to force Git to use non-buffered I/O when
writing to stdout. It can only accept two values, '1' which causes
Git to flush more often and '0' which makes all output buffered.
Make GIT_FLUSH accept more values besides '0' and '1' by turning it
into a Boolean environment variable, modifying the required logic.
Update the related documentation.

Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>

Documentation/git.txt

@@ -724,16 +724,14 @@ 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
+	If this Boolean environment variable is set to true, then commands such
 	as 'git blame' (in incremental mode), 'git rev-list', 'git log',
-	'git check-attr' and 'git check-ignore' will
-	force a flush of the output stream after each record have been
-	flushed. If this
-	variable is set to "0", the output of these commands will be done
-	using completely buffered I/O.   If this environment variable is
-	not set, Git will choose buffered or record-oriented flushing
-	based on whether stdout appears to be redirected to a file or not.
+	'git check-attr' and 'git check-ignore' will force a flush of the output
+	stream after each record have been flushed. If this variable is set to
+	false, the output of these commands will be done using completely buffered
+	I/O. If this environment variable is not set, Git will choose buffered or
+	record-oriented flushing based on whether stdout appears to be redirected
+	to a file or not.
 
 `GIT_TRACE`::
 	Enables general trace messages, e.g. alias expansion, built-in

write-or-die.c

@@ -20,15 +20,12 @@ void maybe_flush_or_die(FILE *f, const char *desc)
 {
 	static int skip_stdout_flush = -1;
 	struct stat st;
-	char *cp;
 
 	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);
-			else if ((fstat(fileno(stdout), &st) == 0) &&
+			if (!git_env_bool("GIT_FLUSH", -1))
+				skip_stdout_flush = 1;
+			else if (!fstat(fileno(stdout), &st) &&
 				 S_ISREG(st.st_mode))
 				skip_stdout_flush = 1;
 			else