added the zero case to be copied instead of hard linked to prevent overuse of hardlink (limited to 2^16)

"As of Linux 3.11, the ext4 filesystem limits the number of hard links on a file to 65,000"

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/ext4/ext4.h?id=6e4664525b1db28f8c4e1130957f70a94c19213e#n229