Fix push not allowed to protected branch if commit starts with 7 zeros.

Created by: cirosantilli

Fix https://github.com/gitlabhq/gitlabhq/issues/8232

This fix works because the function is only called by the internal API, which gets refs from git pre-receive-hook from gitlab shell, which always sends full SHA-1s to the stdin.

There are many other such similar problems on the APP where short 0 strings are tested for: there is even a '0' * 5, which exists in the gitlab repo: only 40 zeroes should be used everywhere: this SHA1 has not been inversed so far and would break Git: http://stackoverflow.com/questions/1902340/can-a-sha-1-hash-be-all-zeroes