wrapper: move is_empty_file() and rename it as is_empty_or_missing_file()

is_empty_file() can help to refactor a lot of code. This will be very
helpful in porting "git bisect" to C.

Suggested-by: Torsten Bögershausen <tboegi@web.de>
Mentored-by: Lars Schneider <larsxschneider@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -690,3 +690,16 @@ int xgethostname(char *buf, size_t len)
 		buf[len - 1] = 0;
 	return ret;
 }
+
+int is_empty_or_missing_file(const char *filename)
+{
+	struct stat st;
+
+	if (stat(filename, &st) < 0) {
+		if (errno == ENOENT)
+			return 1;
+		die_errno(_("could not stat %s"), filename);
+	}
+
+	return !st.st_size;
+}