Github has officially announced that they have over 100 million active users, and over 400 million repositories.
At Github's scale this is a common event; on the order of monthly.
There's a reason Git doesn't simply use CRC32 or the like.
EDIT: Looking further into it, it looks like there's almost two thousand commits on github starting with this exact hash: https://github.com/search?q=hash%3A0000000&type=commits&p=1