Fix #534: 64-bit issues in Windows

Fix #534: 64-bit issues in Windows

off_t is always 32 bits in Windows, which is beyond stupid, but we just
don't care anymore because we're using `git_off_t` which is assured to
be 64 bits on all platforms, regardless of compilation mode. Just
ensure that no casts to `off_t` are performed.

Also, the check for `off_t` overflows has been dropped, once again,
because the size of our offsets is always 64 bits on all platforms.

Fixes #534