Make pack creation always fsync() the result

This means that we can depend on packs always being stable on disk,
simplifying a lot of the object serialization worries.  And unlike loose
objects, serializing pack creation IO isn't going to be a performance
killer.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

write_or_die.c

@@ -78,6 +78,13 @@ ssize_t write_in_full(int fd, const void *buf, size_t count)
 	return total;
 }
 
+void fsync_or_die(int fd, const char *msg)
+{
+	if (fsync(fd) < 0) {
+		die("%s: fsync error (%s)", msg, strerror(errno));
+	}
+}
+
 void write_or_die(int fd, const void *buf, size_t count)
 {
 	if (write_in_full(fd, buf, count) < 0) {