While downloading the sparse git database I got impatient and ran fsck-cache on it. I got...
... missing (null) ff515b91674ff2f5b082a927676dbf392d04d9ce missing (null) ff94be6fbcce10b83824defbdc2f8819121bbd4d missing (null) ffa4ddda950185bf313654e903207a3fc2d5f261
That "(null)" is not from git, but from glibc in response to NULL passed for %s. It might as well crash (as some other libc do) or send in the marines.
Morten