Github doesn't seem to allow searching revision hashes, but presumably Google has most of Github crawled by now, and I didn't spot any commits with that as a label: https://encrypted.google.com/search?num=100&q=badc0de%20site... (more hits than I expected though).
Google doesn't appear to match partial hashes. I searched for a random GitHub commit hash, and it showed up, then removed one character, and it didn't.
Each hex character represents 4 bits. That means that a 7 character string is 28 bits. That's about 268 million possibilities. On average, it would take around 134 million commits to get one that started with "badc0de".