git_fetch_options store flags as a bitfield.

This is a challenge for the Rust wrapper for this library (and possibly other non-C languages), because Rust's FFI doesn't understand C bitfields. Layout of common C types is generally well-known and easy to match, but bitfields are a less common feature, and I don't know how much their layout may vary across compilers, architectures, and endianness.

Could you provide getter and setter functions for manipulating these fields? Or perhaps you could guarantee that the library's ABI will always use the least significant bits of the unsigned int field?